Red shop rags - Soak in a bucket of hot water with dawn overnight. Agitate when I think about it. If really greasy, will drain and resoak for another day.
Microfiber - if they get any type of metal or leaves (we have oak trees and those leaves are like armor), I toss them. Our washing machine has a soak cycle. I add some zote (bar laundry soap) and use a knife to cut off some flakes. Add in a Wisk "pack" (those little plastic balls of cleaner) and let soak for two hours. Usually come out pretty clean.
Note on Zote - I tried this a few months ago and this is really great laundry soap. It's just different, because it comes as a bar and your supposed to run it across stains. I prefer to use a knife and peel off small flakes. My clothes have never been cleaner and leaves a nice "lemon" scent. I picked it up at Walmart.
100% Cotton towels - I have some 100% cotton towels from
Zaino that is use only for vehicle polishing. On these, I only use liquid detergent and dry with no fabric softener. I keep them in plastic bags in the garage for ultimate protection.
For towels that have some really tough stains, I use
Ritz Brightener and Whitener. I'll take a bucket, add the whitener and boil water on the stove. Add it. Let it site overnight. Then dump it into the washing machine, let it add more water and go through the cycle. Very good for stains on colors or whites. For whites only, they have a "white wash" product that is very good. As well, have a stain remover which works very well.
The "brightener and whitener" is also very good for soaking nasty seatbelts in, to get them really clean. I can't even tell you how effective it was on a set of parchment seatbelts I'm restoring.
Oxy [clean] or whatever word you happen to see. I've had absolutely no luck with anything that says "Oxy" on the label. I wouldn't even say it's as effective as just plain unbranded detergent.
Hope that helps.