Ok so a buddy of mine is a diesel mechanic (& a damn good one) & he builds differentials so i trust his judgment. He said if it was his truck he would use 75w140 synthetic in everything, diffs, trans & t-case. Says it has the friction modifiers i need & will be better in high heat situations. The LO says 80w90. Thoughts? Opinions?
So, this is kind of a tricky one. I wouldn't say it's necessarily a bad idea, but it's a specific use case, really. Not to negate or insult your buddy's knowledge and experiences with this, just providing my own based off what I've done with my trucks.
75w-140 is a good recommendation for gear oil in the differential instead of the 80w-90 from the LO if you plan on towing a lot with the vehicle. I say this because I towed a camper 4 hours back and forth one weekend a month for my reserve time and I grenaded a diff that had the factory spec 80w-90 gear oil in it. This is because the 75w-140 has a higher resistance to thinning out at higher loads and temperatures that towing would apply to your vehicle. Unlike the 80w-90 which is a more of an on-road gear oil rated for light towing and automobile use IIRC. 75w-140 also has a much shorter lifespan than 80w-90 because it moves up and down in temperature more broadly and wears out the additives from the factory whereas 80w-90, when it stays in it's operating temperature range, lasts much longer because it doesn't change that drastically in temperature. I now don't tow with the truck that I grenaded the diff on, even though I replaced the rear axle.
In the case of the trans and the t-case, I would still recommend the factory spec of GM DEX2 or 3, but not DEX 6. The DEX fluids being hydraulic fluid instead of gear oil have a higher resistance to foaming and aeration while at operating temperature with the gears moving in and out of the fluid at rapid speeds, whereas gear oil is kind of quick to foam even in a differential. I'll link Banks' video of "what is going on behind your stock diff cover"
here for viewing that goes more in depth into the practice more than the theory.
Is your trans in the 1028 the THM 400 or do you have a manual gearbox in the truck? I would recommend manufacturer fluid for the trans regardless of what the make of the transmission is. An 86 Chevy NV4500 may have a tendency to splinter gears more often without GM manual trans fluid from that era that had say a higher concentration of copper for instance that would fill in the chips and nicks of the gears of the transmission.
These are just my thoughts and experiences, though. I would also throw in my recommendation of going synthetic whenever possible, since there are more additive protectants and detergents usually. Synthetic also has a longer lifespan than conventional fluids in almost every application. I hope this helps.
