I would not run ATF. Just too thin for a manual tranny and not much antiwear additive. My '95 F150 never shifted right until I ran the next thicker fluid, which would be a syncromesh fluid. My choice is Redline MTL, which has tons of antiwear additive. Shifted beautifully with MTL.
ATF fluids contain a specific additive package for Automatic Transmissions, and not for manual transmissions. Most ATF specifications for use in mtl's, IMHO, were to improve cold weather shifting, and not to improve wear.
The above quote is from someone who had blended oil and transmission fluid (I actually ran his fluid in the F150 tranny at one time): Quote is from 5th post here.
The Getrag 5-speed in my old BMW could take ATF, straight-30 motor oil or 80W gear oil according to the owners manual. Mercedes owners have told me their cars manual transmissions were also not picky. Redline MTL is what I settled on.
OK, got some coastal type f atf in the trans this weekend. So far, seems to be Working quite a bit better than before. Still not like new but very livable.
Call Redline (number is at redlineoil.com) and talk to their chemist, Dave. He is very accessible. Tell him the problem you are having with going into gears and see what he recommends. My gut feeling is that MTL would work better, but what do I know. No harm talking to him.
Thanks for the replies! I thought it would be interesting to note that my trans has some wear. You have to be at very low speed or stopped to go into second (I take off from a stop in second). Sometimes it will grind a little when I turn at low speed trying to get it into second. This is a grey area because a lot of the times I am not moving fast enough to use 3rd gear in some turns. Any additives? Certain brands? Lucas? Royal purple? I am thinking ill probably just go with the motorcraft but I'm hoping like heck there is something better for my tranny with some wear.
A common problem with older transmissions was that the sychros were made from brass or bronze. 80W-90 gear oil was the common recommendation. This was not optimum, but worked ok with the old API GL-4 and earlier oils. Now, GL-5 EP hypoid oil is all that is commonly available in that weight. GL-5 contains a higher dose of sulphur-based additive (the stinky stuff in gear oil) to better protect rear axle gears . It also corrodes yellow metals, making the syncros go away. This is not a problem in non-synchronized heavy duty truck transmissions, so these oils also get the API MT-1 designation. People mistakenly think MT-1 means its safe for all manual transmissions.
This debate rages on old Volkswagen forums because those cars have the transmission and differential in a single housing. What is specified in the owners manual sometimes doesnt exist, so people believe newer specs are always backward compatible. Since the ring and pinion gears are spiral bevel, like pre-1950's Fords, not hypoid, EP gear oil is unnecessary and in fact harmful. This is a similar issue to newer low-zddp motor oils in older flat tappet engines.
After running the Pen syncromesh for about 7 months, I decided to go thicker because I still had a good amount of whine in gears 1 through 3 but shifting was much better compared to any ATF.
This may be controversial, but I just put in Redline MT-90 which is a GL-4 synthetic manual trans oil with the viscosity of 75w-90. Shifts even better than the syncromesh and doesn't feel "too thick" at all. Plus it is very good for the yellow metal syncros, and has a just right coefficient of friction to make them work when shifting.
The Pennzoil came out dark in that short time, so it may be too harsh of an environment for it making it oxidize, the redline should be able to handle the heat better. Time will tell if its too thick for winter shifting, but I'll use it until I run into any issues.