Use 80-90 gear lube with the GM posi additive. If you don't use the additive then the clutches wear and the posi will clunk and make noise. I added it to my '72 and it quieted down and works much smoother.
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'51 South Bend Model "9A"
'56 Chevy 3600 NAPCO
'74 Chevy Cheyenne Super K20-restored on 37's
'72 GMC Sierra Custom K2500- in progress
'08 GMC Envoy Denali
'12 GMC Sierra 2500HD Denali
'17 GMC Yukon XL
VCCA member #58596
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