GM 88900399 Synchromesh Friction-Modified Manual Trans Fluid 32oz

This is the genuine GM synchromesh manual transmission fluid, part number 88900399 (the current number for the fluid GM previously sold as 12345349 and 12377916). It is a semi-synthetic, friction-modified lubricant, not a heavy 75W-90 gear oil. The friction modifier is the whole point: it lets the brass synchronizer rings grab and slow the gears the way GM engineered them to, instead of skating and clashing the way they do on thick hypoid oil.

The GM boxes that call for it are the New Venture NV1500 and NV3500 five-speeds found behind four- and six-cylinder S10, Sonoma, Blazer and full-size trucks, along with the NV3550 used in some applications. Many builders also run this fluid in aftermarket and import five- and six-speeds that specify a friction-modified synchromesh oil, including certain Tremec, T5 and Getrag units. Always confirm your gearbox actually calls for a synchromesh-type MTF before you switch off gear oil.

On the bench or under the truck the job is simple: warm the box, pull the drain plug, then fill through the fill plug on the side of the case until fluid reaches the bottom of the hole. Most of these transmissions take roughly two to two and a half quarts, so plan on more than one bottle for a full service. Fill it level, not by the quart count on a sticker, since capacity drifts with the bellhousing angle.

If your manual has gone notchy shifting into second when cold, grinds going into reverse, or feels like it fights you the first mile of a winter drive, running the wrong fluid is usually the reason. Dumping the old, contaminated oil and refilling with the correct friction-modified synchromesh fluid is the cheapest fix on the list and often the only one you need.