Well, I decided to just try a few different things for myself.
First, I switched over to the Royal Purple Synchromax. At first it seemed to make the transmission a little quieter, but over time it seemed to end up just as noisy as before with the Merc V that I believe I had in it before. However, the shift quality was a night and day difference. It took noticeably less effort to shift, I could shift a little quicker, and it was less clunky. Definitely a great improvement.
After a few hundred miles driving with the RP, I decided to try the 0w-20 full synthetic engine oil I've heard about some people using. I made sure to save the RP stuff in a clean container in case I want to switch back. Anyway, with the 0w-20 in there (I used Mobil 1 synthetic, non high mileage), it shifts almost as good as the RP did. Probably just a tad less smooth. But still, much much better than the original fluid that was in there. The 0w-20 seems to quiet down a bit of the noise the trans would make, but only during light load like cruising. I don't really know if I would be able to fully get rid of that noise, I know my transmission probably just needs to be rebuilt, but I do find it interesting that the 0w-20 seems to have had the best impact on the noise so far. Also, for what it's worth, I've read some people use 0w-30 instead of 0w-20, but I thought that might be too thick compared to ATF. If I remember correctly, I had read that ATF is around 10w, so I figured 0w-20 was enough of a jump in viscosity as is. But part of me is curious how 0w-30 would compare in the trans to 0w-20.
One more thing I would like to try is the royal purple synchromax with half a quart to a quart of the lucas trans stop slip. I know it was recommended earlier on this thread to add some of the stop slip, and I've come across it a few other times where people claim it helped their manual as well. I'm a little more skeptical of using that, because I'm not a fan of using bandaid fluids like that since all they do usually is increase the viscosity of the fluid. But when it comes to using ATF, you don't have many other ways to do so. Anyway, I'm going to run the 0w-20 for a while and try that combination next for comparison. Now that I've tried a couple of these fluids myself - and also reading some of the previous replies - I'm starting to think you can use just about anything in a manual trans as long as the viscosity is about right and the oil isn't going to harm the materials inside the trans lol.
I just want to say again, I'm not recommending everyone go swap out their trans fluid to whatever the hell they want. I just wanted to share my experience trying other fluids just for my own curiosity, and hear what other people have tried and their experiences as well.
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