TheDrivenMan

I am a transmission guy. I sell them, build them and install them. Been on Overhaulin' and The Power Block messin' with 'em. That said, you should run Synchromesh in your transmission. Not the expensive Synchromesh synthetic blend either, just the natural regular old Synchromesh. Here is why: Unlike your engine, where more lubricity and detergent is generally a good thing, a transmission is designed around it's fluid as a moving part. The synchros must sling just enough fluid during shifting to make the process smooth and efficient. Too much sling and you get slow shifts, too little and the gears grind. That is not to say that there are no other fluids that will do the job, but the 3650 was made for Synchromesh and without the properties of other fluids available you might just end up with an issue.

I have customers call all the time with shift quality issues. The first thing I ALWAYS ask is what fluid they put in. Generally that is followed by "drain that crap out and put in what the transmission calls for". Lots of $40 bills running into catch pans because everyone assumes that the fluids that the factory spent thousands of dollars to develop is not as good as the stuff a mom and pop operation can pump into a bottle.

The good news is, unless you have pure synthetic in the case for an extended time, you should be able to drain the wrong stuff out and put the right stuff in without doing any longterm damage to the transmission. Pure synthetic can over saturate your seals. When putting natural back in the seals can shrink and leak. 'nuff said. Hope this helps.