I also have a 5w30 number on the dipstick/plug. It's all I've ever put into it, and I've never had any problems shifting. I have a short shifter and my 1-2 & 3-4 shifts can be done with 2 fingers. The select (left-right shift) cable has a bit of friction though, but I deal with it for now. The car has almost 182,000 miles.https://www.youtube.com/watch?v=MXsRfbOiBhE
- I've referenced this video a few times to help aid in the description. He also shows an old school synchro vs a T5.
The way synchros work is they are basically cones with teeth. The cones slow the mating gear down via friction (and they also have some grooves to bite through whatever lubricating fluid is specified), and the teeth help guide the ring (forget the exact name) that delivers power from the input shaft to the intended gear. With the wrong fluid the synchros would have trouble stopping the gears for this change to properly occur. Gear oil being much heavier than engine oil would probably cause this issue, so you may be able to resolve it by attempting some sort of flush.
I don't know what solvents would cause any sort of damage to the synchros (often times they're something like brass, or have some sort of fiber lining), so I'd look into that, 282 is around the same era as the T5 debut, so I wouldn't make any guesses at a chance of ruining the synchros.
[This message has been edited by Doober (edited 09-16-2017).]