12 bolt posi oil / Muncie 4 spd oil recommendations

[JST69Z28]

Does the team have a recommendation for gear oil for a 12 bolt posi ('69 Z28) and for a Muncie 4 spd?  I have read several posts about using GL4 instead of GL5 for the Muncie due to synchro wear, is that accurate?  Is there a recommended brand or two? 

For the diff, does it need to be GL4 or 5, does it matter? In most stores, all you find is GL5.  I will add the GM limited slip additive to whatever oil I get.  Thanks.

[David K]

Whatever the manual recommends is what I would use. Rear I think is 80w/90 and don’t forget the additive bottle for the clutch pack.

[Mike S]

Hi JST,

  I've researched GL4 vs. GL5 a lot and decided to use GL4 in my manual and differential.
Here is a link to a recent 2022 article that compared the two. https://oilsadvisor.com/gl4-vs-gl5/
 Here is an excerpt from that article which I have read elsewhere before when it comes to 'yellow' metal (ex: brass syncros):

  Using GL5 oil instead of GL4 can damage your gear parts because of the high levels of sulfur and phosphorous that reacts against the yellow metals, causing corrosion. This greatly reduces your gear’s life and performance.

  For my Muncie and posi rear I use Sta-Lube 80W-90 made by CRC (P/N SL24229) which a mineral based oil for GL4 applications. For the posi I also add a 4oz bottle of Delco limited slip lube (AC/Delco P/N 10-4003). The way most of us drive (aka: pamper) our cars, this oil should be just fine.

Mike

[MO]

Good info Mike.

[JST69Z28]

Thanks for the feedback.