big_how

I've got an 85 with the Doug Nash tranny, and it leaks (those things leaked on the showroom), and I was wondering what kind of fluid I should put back into the tranny because in my service manuals (even the GM one), it says that they don't need to be serviced. Can I do a drain and refill if I wanna?

bills1985corvette

The overdrive section, mounted to the rear of the T10 4 speed used ATF (Dextron III). I use Mobile I ATF and have had no problems with my 1985. I believe the T10 uses 75-90 gear oil. I have read you should not use synthedic in this unit. I change the fluid in the rear axle and T10 unit every 4 years (20,000 miles max). I chenge the fluid in the overdrive unit every Spring.

c4cruiser

While the Owner's Manual doesn't really show a maintenance schedule for the OD unit, it should have the fluid changed every 12-15K miles for longevity. GM Dexron II/III is the proper fluid and it holds just over two quarts if you also blow the fluid out of the cooling lines. It's not a good idea to use syntentic fluids in either the 4-speed or the OD unit. 75-90W gear oil is fine for the T-10 4-speed portion.

The OD unit has a brass filter that should be changed at each fluid change. It is the same filter that was used on Ford "FMX" trannies found on late 60's to mid 70's Cougars, Torinos, some Mustangs and some F150's. Carquest has the filter under part number 85926. You can also cross-reference this number to some other filter brands.

Use RTF black sealant on the tranny pan to seal it. Make sure the pan rails are clean of all grease and oil and make sure the pan rails are straight. Don't tighten the bolts too tight; 10-12 ft-lbs is fine but just make sure they are all equally tight.