Real truck transmissions have big gears with lots of inertia. The super-slippery synthetic lubes don't let the synchronizer rings get a good grip on them to match gear speeds as a shift is made. Straight mineral oil such as the Ford N-series tractor transmission lube sold by Tractor supply is a good one to use, plus it's a lot less expensive to buy. I believe it has a "GL-1" rating.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!