In an effort to help those of you who was where I've been, trying to wade through all of the information about what oil(s) work in the G-56 and decide what oil or combination of oils is best to run in my G-56. I thought I'd post up the results of some real world testing to save you some headache, unnecessary wear & tear, and some $$$ out of your pocket.
After reading the G-56 Factory fill is NOT ATF+4thread
I decided to change out my Original ATF+4 that had 25K-28K miles on it with the Mobile 1 Delvac Synthetic SAE50 & Amsoil Synchromesh mixed at a ratio of 5 qts of Delvac to 3 qts of synchromesh, (62.5% Delvac/37.5% Synchromesh). The Delvac & the Amsoil had similar bases and Pensoil & Royal Purple seed to have mediocore to bad reviews so that's why I went with this combo. I Also went with a combo instead of straight Delvac to help with hard shifting which many guys mentioned especially in cold weather even though I'm in So Cal where it never really gets below 30 degrees in the winter.
I also wanted to overfill it with 7 qts + to help with bearing cooking when pulling loads up hill so I bought a TDS custom designed inspection plate with pre-drilled nipples for a trans cooler, and a higher fill point that allows for 7.5 qts to be pumped in with out taking apart the cab and pulling the shift boot. I designed these but you can buy them like this from TDS, it makes overfilling & adding a cooler much easier.
At the time I made the switch I had a stock clutch and right from the get go the shifting was difficult, notchy, near impossible to get in to 2nd from dead stop most times, third was hard to get it in to also. Eventually I upgraded to a Valir Street Quiet DD, and the "gear roll over" was what was to be expected but even Valir recommends the Delvac 50 straight no mix. After about 10K with the new clutch I began having issues getting the trans out of gear in both 2nd & third and getting it in to reverse was getting harder and getting it out of reverse could ONLY be done while rolling, so it looked like things where getting worse.
At this point I decided on the recommendation of Danderson who had been where I was to switch the trans fluid to the Amsoil 75/90 Manual Transmission Gear Lube (75/90 MTG) Post #775
When I drained the Delvac & Synchromesh with about 32K to 35K miles on it and to my surprise there where no real big chunks but it did what I would consider a lot of fine brass in the oil, see for here are some pics of the bottom of the pan
After the swap to the Amsoil I drove it 10 miles and shifting in to 2nd was much smoother, I didn't have to fight it, even at a stop, shifting in to 3rd was good, it still hangs coming out of 3rd & reverse but I think that is wear from the previous Delvac oil, I did notice much reduced "gear roll over" when lugging it, with the Delvac anything under 1300-1400 it would chatter and start to shake especially when pulling load in to a taller gear. With the Amsoil 75/90 I could lug it down to 900 RPM and the gear roll over was much reduced with very little chatter and no vibration. to me this is much improved performance over the Delvac/Synchromesh combo. The MTG is lighter viscosity almost like vegetable oil VS the Delvac/synchromesh that was closer to hot honey.
I do run a temp gauge and on long trips, it takes the Delvac/Synchromesh about 20-30 miles to get to 100*-120*, 50-60 miles to get to 140*-150*, and 200 miles to get to 180*. Wit the MTG it took only 10 miles of street driving to get to 120*. I'm driving 600 miles through the desert towing 4K this weekend up & down grades I'll watch the temps VS mileage and report back.
My initial reaction is the the MTG works much better but will most likely need more frequent service intervals.
And lastly Price; I could ONLY get the Delvac in a box of 4 gallons for $175 plus I had to buy the 3 Amsoil Synchromeshes which ran me at least $36-$40, so I was in the Delvac/Synchromesh $110 for 8 qts and I'm in the MTG $123 for 8 quarts at dealer price (Art 510-258-1071). So I think I'll end up spending more on the MTg since I suspect the service intervals will also be shorter but I think it is doing a lot less damage to my brass synchros which in the long run will more than make up for the little extra spent on better working oil.
Good Luck hope this helps you avoid a lot of wasted time & money chasing your tail for the right trans oil for the G-56, like I did.