Gear Oil (Page 3/3) USMUCL FEB 28, 02:41 PM If you look at the descriptions on the bottles, GM Synchromesh uses the terminology "semi-synthetic" while Pennzoil Synchromesh does not. I realize that this might come down to marketing, and they could be the exact same fluid. But, I always wondered if there was a chance they were different and thus had different coefficients of friction. olejoedad FEB 28, 02:53 PM To answer the O/P question.... Several years ago, I drained the OEM fluid from my 86 four speed transmission and refilled it with Mobil ONE 5w-30. There was a noticible improvement in shifting ease. The transmission was butter smooth. It was in there for an additional 100,000 miles. I saw no need for going to a Synchromesh type fluid. 2.5 MAR 02, 11:48 AM I've got Redline's equivalent "MTL" in mine. Its full syn.