12 Bolt Rear End with Posi What Oil to use? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

petey-pablo · 08-27-2009, 07:49 AM

Ok just like the title says I have a 12 Bolt with Posi. What oil should I use on my rear end? Is regular oil ok or can I use Synthetic what Weight? Also Should I change it, the truck has been sitting for over a year and leaked alittle just enough to leave a small quarter size stain..How hard is this to do for someone who's never done this job before? Also since I will have the differential cover off how can I tell what gears I have?
Thanks Pablo...........

crawdadfox · 08-27-2009, 07:56 AM

Use a quality standard gear lube along with GM limited slip additive. When you have the cover off there will be numbers stamped on the side of the ring gear...like 41:10...divide the smaller number into the larger one and that will give you your axle ratio, its easier than counting the teeth on the ring and pinion.

rwolf9653 · 08-27-2009, 08:01 AM

To tell what gears you have you can count the teeth on the ring gear and count the teeth on the pinion gear. Divide these two numbers and you get you ratio.

As for the gear oil you can use regular 80W90 as long as you put in the additive for the positraction. I prefer to use Amsoil.

cdowns · 08-27-2009, 08:47 AM

royal purple no additives needed for posi units

**Green Machine** · 08-27-2009, 08:58 AM

We use Royal Purple synthetic without any additives. It has mild posi snap/chatter going around corners in town. Our Son (19) drives it and likes the mechanical feel of the posi sticking so we have not put in any additives. Royal Purple bottle states that it is for posi rear ends, so the lube should be ok.

smokekiki · 08-27-2009, 12:24 PM

I run Royal Purple with Ford limited slip additive. Without the additive and new discs in my LSD it was chattering.

*vegaschevy* · 08-27-2009, 01:15 PM

I run wal mart brand gear lube with GM additive. It has all the qualifications and specs and passes GREAT with tests.

Ackattack · 08-27-2009, 01:33 PM

I know my new Eaton posi said specifically not to use synthetics.

also, counting the pinion gear teeth sounds easier than it is. It's basically blocked from view by the diff. Look for the stamp on the ring gear.

piecesparts · 08-27-2009, 02:18 PM

I have run synthetic lube with my Eaton posi, but switched back when we got to reading further in the data sheets. I always use the additive, since the dealer put lube in my 95 Impala SS without the additive and it chattered really bad. Small price to pay for protection. I have Eaton posi rearends in all of my trucks.

mr48chev · 08-27-2009, 02:48 PM

Good quality 80-90 and unless it specifically says that it is made for limited slip aka posi run the posi additive.

lilred67 · 08-27-2009, 04:51 PM

i run mobil 1 85/90 full synthetic with gm diff. additive. mine chattered without it.....

70c10 · 08-27-2009, 07:17 PM

The directions for my eaton posi said not to use synthetic.

petey-pablo · 08-27-2009, 10:45 PM

I was going through the copies of some receipts I got from the PO and I found one from a Local Speed Merchant Dated FEB/16/06 that apears to be for the rearend posi unit it reads "EAT19556 12T GM HD 3.73 & POSI" Is this a good unit? I think I should plan on getting a diff cover with a drain plug as well to make this easier
Thanks guys

capev86 · 08-27-2009, 11:11 PM

they didn't really have much for synthetic fluids when these axles were being built. the additive certainly can't hurt if you have a limited slip. but the rear diff gets pretty hot, so the synthetic is a good idea especially if you work your truck. mobil 1 and others make economical gear lube that is good for an average joe car, but i will be using royal purple gear 80w-90 in my 72 burb 3/4 ton with HO52 axle when i change to a 4.10 gear.....hopefully soon. already have royal purple manual trans fluid in the saginaw 3 on the tree. its worth the money for a vintage truck that will be used as a truck.....aka.....heavy duty towing on occassion! my dad put royal purple in the '84 burb 10 bolt when he swapped it into his 87 burb (3.73 gears are soo much better than 3.08)

piecesparts · 08-28-2009, 11:16 AM

Quote:

Originally Posted by petey-pablo

I was going through the copies of some receipts I got from the PO and I found one from a Local Speed Merchant Dated FEB/16/06 that apears to be for the rearend posi unit it reads "EAT19556 12T GM HD 3.73 & POSI" Is this a good unit? I think I should plan on getting a diff cover with a drain plug as well to make this easier
Thanks guys

I have been running an Eaton rear end posi unit in my 84 GMC for the beter part of 12 years. It was in the truck when I purchased it and since then the speedo has turned over twice. You have a good unit. You are right when Eaton built these in the early years there was no such thing as synthetic oils, but Eaton stilkl exists and the info is from the recent years, they do not like a synthetic in their units. I have run synthetic in one of mine (91 daily driver with 275,000+ miles) and have had no issues, though.

capev86 · 08-28-2009, 11:57 AM

yeah....use good fluid and they last a long time.....not good if you are in the business of making money on repairs. regular fluids don't take the heat and abuse like synthetics.....the rear axle is one place that gets pretty warm and is often neglected during maintenance cycles.

Coursey · 07-03-2015, 11:34 AM

I am not sure what type of oil is in the rear end. It is a posi. What type of oil is safe to top off the oil level?

**'68OrangeSunshine** · 07-03-2015, 04:35 PM

Any good gear oil, SAE 90 [or a multi-viscousity in that range] that is rated GL-5 for limited slip. I also add the GM Posi ''Whale Oil'' additive.

67 chevelle · 07-03-2015, 05:15 PM

Quote:

Originally Posted by smokekiki

I run Royal Purple with Ford limited slip additive. Without the additive and new discs in my LSD it was chattering.

The ford additive is what I use also ,,, man does it stink ,, it has solved "issues" for me ,, clunking around corners

KMAT · 01-03-2019, 02:42 AM

Will it damage anything if I use Carquest 80 90 mixed with partsmaster 8090 and some trans-x posi-trac lube

**Ironangel** · 01-03-2019, 05:08 AM

Shouldn't hurt anything but why mix? The 12-bolt only holds like 1-3/4 quarts so 2 quarts of any MP GL-5 Gearlube with 4-6 oz of posi additive your good to go. FelPro cover gasket #13391. Pop the cover and inspect the old fluid for fleck and friction plate pieces, busted ring gear teeth, etc...If everything looks good button it back up and refill with 80w90. Mix the full 4oz bottle of posi additive with the first quart of gear lube and squirt it all in. Fill until lube begins to run out the fill hole and install the plug. On old factory Posi units that were broken in with mineral/dino oil, I use the old school mineral oil because thats whats in those frictions. New units with the carbon fiber frictions it's ok to use synthetics. I use this https://petroleumservicecompany.com/...-1-quart-case/ with this in my posi's https://www.summitracing.com/parts/a...SABEgKSJvD_BwE never had an issue yet.

08-27-2009, 07:56 AM	#2
crawdadfox Registered User Join Date: Aug 2008 Location: New Orleans Louisiana.....a great place to visit but you wouldn't want to live here!!! Posts: 883	Re: 12 Bolt Rear End with Posi What Oil to use? Use a quality standard gear lube along with GM limited slip additive. When you have the cover off there will be numbers stamped on the side of the ring gear...like 41:10...divide the smaller number into the larger one and that will give you your axle ratio, its easier than counting the teeth on the ring and pinion. __________________ Down......with the sickness!! Last edited by crawdadfox; 08-27-2009 at 08:00 AM. Reason: add

08-27-2009, 08:01 AM	#3
rwolf9653 more ideas than money Join Date: Dec 2008 Location: spring grove, pa Posts: 1,068	Re: 12 Bolt Rear End with Posi What Oil to use? To tell what gears you have you can count the teeth on the ring gear and count the teeth on the pinion gear. Divide these two numbers and you get you ratio. As for the gear oil you can use regular 80W90 as long as you put in the additive for the positraction. I prefer to use Amsoil. __________________ Randy 1948 Chevy 3100 5 window 1966 Chevy C30 Dually 1967 Chevy K10 1972 Chevy K20 1972 Chevy C10 1972 Chevy K5 Blazer 1987 Chevy V10 2003 Chevy Suburban

08-27-2009, 08:47 AM	#4
cdowns Senior Member Join Date: Jan 2003 Location: daytonabeach Posts: 22,956	Re: 12 Bolt Rear End with Posi What Oil to use? royal purple no additives needed for posi units __________________ 71c-10 350/2004r/4:11 lowered3/4 longbed/dead by hurricane MEANING OF DEATH::::: SOMEBODY ELSE GETS YOUR STUFF DONT BELIEVE EVERYTHING YOU THINK TAKE MY ADVISE;I DON'T USE IT ANYWAY

08-27-2009, 08:58 AM	#5
Green Machine Senior Member Join Date: Jul 2004 Location: Estherville, Iowa Posts: 3,371	Re: 12 Bolt Rear End with Posi What Oil to use? We use Royal Purple synthetic without any additives. It has mild posi snap/chatter going around corners in town. Our Son (19) drives it and likes the mechanical feel of the posi sticking so we have not put in any additives. Royal Purple bottle states that it is for posi rear ends, so the lube should be ok. __________________ 1968 C10 307 3spd Long Fleet ------ http://67-72chevytrucks.com/vboard/s...d.php?t=201103 1970 C10 305 Super T10 Long Fleet --- http://67-72chevytrucks.com/vboard/s...d.php?t=202285 1971 C20 383 TH350 Dana Posi ----- http://67-72chevytrucks.com/vboard/s...d.php?t=206894 2001 GMC Sierra 1500 C3 6.0

08-27-2009, 12:24 PM	#6
smokekiki Mike Join Date: Jan 2003 Location: west chester pa Posts: 2,474	Re: 12 Bolt Rear End with Posi What Oil to use? I run Royal Purple with Ford limited slip additive. Without the additive and new discs in my LSD it was chattering. __________________ 70'c/10, 71 suburban4x4 402bb, 72suburban 4/6 drop, 72k/5 4x4 blazer 4" lift 35 tires

08-27-2009, 07:49 AM	#1
petey-pablo "THE GREEN GOBLIN" Join Date: Mar 2008 Location: Fremont Ca. Posts: 1,025	12 Bolt Rear End with Posi What Oil to use? Ok just like the title says I have a 12 Bolt with Posi. What oil should I use on my rear end? Is regular oil ok or can I use Synthetic what Weight? Also Should I change it, the truck has been sitting for over a year and leaked alittle just enough to leave a small quarter size stain..How hard is this to do for someone who's never done this job before? Also since I will have the differential cover off how can I tell what gears I have? Thanks Pablo........... __________________ Petey-pablo "THE GREEN GOBLIN" 1972 Cheyenne 10 Short Fleet Side Front 2.5inch Spindles with 2inch Springs Rear 4inch drop Springs. 454BBC TH400,Posi 1971 Cheyenne 10 Long Fleet Side 454BBC TH350,Posi Sold Drive it like you stole it! Petey-pablo@sbcglobal.net

08-27-2009, 01:15 PM	#7
vegaschevy Account Suspended Join Date: May 2009 Location: las vegas nv Posts: 1,493	Re: 12 Bolt Rear End with Posi What Oil to use? I run wal mart brand gear lube with GM additive. It has all the qualifications and specs and passes GREAT with tests.

08-27-2009, 01:33 PM	#8
Ackattack Senior Member Join Date: Aug 2002 Location: Valley Center KS Posts: 3,525	Re: 12 Bolt Rear End with Posi What Oil to use? I know my new Eaton posi said specifically not to use synthetics. also, counting the pinion gear teeth sounds easier than it is. It's basically blocked from view by the diff. Look for the stamp on the ring gear. __________________ Project Bruiser: 68 Pro-Touring C-10 5.3L / 4L60E Project Ironhide: 97 GMC Crew Cab 4x4 Cummins SOLD: '69 GMC 1/2T SWB TBI'd: Project No Buck Assembly Manual Index Sign up Thread!!! Factory Service Manual Index

08-27-2009, 02:18 PM	#9
piecesparts Parts and more parts Join Date: Nov 2003 Location: Lebo, Kansas (middle of nowhere Posts: 6,821	Re: 12 Bolt Rear End with Posi What Oil to use? I have run synthetic lube with my Eaton posi, but switched back when we got to reading further in the data sheets. I always use the additive, since the dealer put lube in my 95 Impala SS without the additive and it chattered really bad. Small price to pay for protection. I have Eaton posi rearends in all of my trucks. Last edited by piecesparts; 08-27-2009 at 02:19 PM.

08-27-2009, 02:48 PM	#10
mr48chev Registered User Join Date: Jul 2007 Location: Toppenish, WA Posts: 15,267	Re: 12 Bolt Rear End with Posi What Oil to use? Good quality 80-90 and unless it specifically says that it is made for limited slip aka posi run the posi additive.

08-27-2009, 04:51 PM	#11
lilred67 Senior Member Join Date: Mar 2005 Location: weatherford,texas Posts: 1,003	Re: 12 Bolt Rear End with Posi What Oil to use? i run mobil 1 85/90 full synthetic with gm diff. additive. mine chattered without it.....

08-27-2009, 07:17 PM	#12
70c10 67-72 Addict! Join Date: Feb 2002 Location: The TAX State! Posts: 7,857	Re: 12 Bolt Rear End with Posi What Oil to use? The directions for my eaton posi said not to use synthetic. __________________ Jim 1970 C/10 Fleetside w/Ghost Flames Lowered 4.5" front and 4" rear (Raked) 355/350 Turbo w/shift kit 10" Redneck Performance Verter w/2500 stall Hooker Super Comps part#2808-1 Performer RPM Air-Gap 12 Bolt w/3:73 gears- Eaton Posi Comp Cams XE262 with 1.6 Crane Energizers, Road Demon 625 and Brodix IK 180 heads

08-27-2009, 10:45 PM	#13
petey-pablo "THE GREEN GOBLIN" Join Date: Mar 2008 Location: Fremont Ca. Posts: 1,025	Re: 12 Bolt Rear End with Posi What Oil to use? I was going through the copies of some receipts I got from the PO and I found one from a Local Speed Merchant Dated FEB/16/06 that apears to be for the rearend posi unit it reads "EAT19556 12T GM HD 3.73 & POSI" Is this a good unit? I think I should plan on getting a diff cover with a drain plug as well to make this easier Thanks guys __________________ Petey-pablo "THE GREEN GOBLIN" 1972 Cheyenne 10 Short Fleet Side Front 2.5inch Spindles with 2inch Springs Rear 4inch drop Springs. 454BBC TH400,Posi 1971 Cheyenne 10 Long Fleet Side 454BBC TH350,Posi Sold Drive it like you stole it! Petey-pablo@sbcglobal.net

08-27-2009, 11:11 PM	#14
capev86 Registered User Join Date: May 2009 Location: Eliot, Maine Posts: 1,314	Re: 12 Bolt Rear End with Posi What Oil to use? they didn't really have much for synthetic fluids when these axles were being built. the additive certainly can't hurt if you have a limited slip. but the rear diff gets pretty hot, so the synthetic is a good idea especially if you work your truck. mobil 1 and others make economical gear lube that is good for an average joe car, but i will be using royal purple gear 80w-90 in my 72 burb 3/4 ton with HO52 axle when i change to a 4.10 gear.....hopefully soon. already have royal purple manual trans fluid in the saginaw 3 on the tree. its worth the money for a vintage truck that will be used as a truck.....aka.....heavy duty towing on occassion! my dad put royal purple in the '84 burb 10 bolt when he swapped it into his 87 burb (3.73 gears are soo much better than 3.08) Last edited by capev86; 08-27-2009 at 11:15 PM.

08-28-2009, 11:57 AM	#16
capev86 Registered User Join Date: May 2009 Location: Eliot, Maine Posts: 1,314	Re: 12 Bolt Rear End with Posi What Oil to use? yeah....use good fluid and they last a long time.....not good if you are in the business of making money on repairs. regular fluids don't take the heat and abuse like synthetics.....the rear axle is one place that gets pretty warm and is often neglected during maintenance cycles. Last edited by capev86; 08-28-2009 at 11:59 AM.



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07-03-2015, 11:34 AM	#17
Coursey Registered User Join Date: Aug 2013 Location: Kentucky Posts: 549	Re: 12 Bolt Rear End with Posi What Oil to use? I am not sure what type of oil is in the rear end. It is a posi. What type of oil is safe to top off the oil level? __________________ 1961 Biscayne http://67-72chevytrucks.com/vboard/s...90#post6978790 SOLD 1971 C-10 http://67-72chevytrucks.com/vboard/s...d.php?t=656258SOLD 1966 Suburban http://67-72chevytrucks.com/vboard/s...85#post7323885

07-03-2015, 04:35 PM	#18
'68OrangeSunshine Senior Member Join Date: Feb 2002 Location: Tucson, AZ USA Posts: 7,022	Re: 12 Bolt Rear End with Posi What Oil to use? Any good gear oil, SAE 90 [or a multi-viscousity in that range] that is rated GL-5 for limited slip. I also add the GM Posi ''Whale Oil'' additive. __________________ Every 25 years I like to rebuild that 292, whether it needs it or not.

01-03-2019, 02:42 AM	#20
KMAT Registered User Join Date: Oct 2018 Location: Sioux center, IA Posts: 81	Re: 12 Bolt Rear End with Posi What Oil to use? Will it damage anything if I use Carquest 80 90 mixed with partsmaster 8090 and some trans-x posi-trac lube

01-03-2019, 05:08 AM	#21
Ironangel Senior Member Join Date: Feb 2010 Location: Falls City, Nebraska "100 Miles From Nowhere" Posts: 2,219	Re: 12 Bolt Rear End with Posi What Oil to use? Shouldn't hurt anything but why mix? The 12-bolt only holds like 1-3/4 quarts so 2 quarts of any MP GL-5 Gearlube with 4-6 oz of posi additive your good to go. FelPro cover gasket #13391. Pop the cover and inspect the old fluid for fleck and friction plate pieces, busted ring gear teeth, etc...If everything looks good button it back up and refill with 80w90. Mix the full 4oz bottle of posi additive with the first quart of gear lube and squirt it all in. Fill until lube begins to run out the fill hole and install the plug. On old factory Posi units that were broken in with mineral/dino oil, I use the old school mineral oil because thats whats in those frictions. New units with the carbon fiber frictions it's ok to use synthetics. I use this https://petroleumservicecompany.com/...-1-quart-case/ with this in my posi's https://www.summitracing.com/parts/a...SABEgKSJvD_BwE never had an issue yet. __________________ Michael of the clan Hill, "Two Seventy Two's" 71 1-ton Dually 350 4-Speed 71 C/50 Grain Truck, 350 Split-Axle 4-Speed 02 3/4 ton Express 14 Indian Chief Vintage 1952 Ford 8N, "Only Ford Allowed On The Property" "Be American, Buy American"

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