Gear Oil 80-90w or 75-90w?

[Mike M95 ex cab4x4]

I have a 1995 K1500 5.7L ext cab. The owners manual calls for 80-90w gear oil in front and rear axle but I have read that 75-90w will improve gas mileage and it's better in cold weather. Has anyone used 75-90w in place of 80-90w without any issues? I plan on using Amsoil Severe Gear either way. Thanks in advance.

 

[GMCTruck]

I have used both in my 1995 GMC K1500. (75w-90 in the rear end only) Can't say I noticed any difference but I also didn't expect better gas mileage. I typically get 18 MPG on the highway (using imperial gallons to calculate) My mechanic replaced the rear end bearings a few years ago and he put in 75W-90, but several months ago, I had to replace an axle bearing so just went back to 80W-90. 80W-90 has worked fine up here in Alberta/Saskatchewan for the past 26 years.

 

[454cid]

Gm switched to the lighter synthetic eventually in 99.

 

[Schurkey]

Perhaps a moderator will move this thread to Axles and Brakes. I don't know why you'd post it in Engine Performance.

Do you have G80 Gov-Lock?

My G80 in a 14-bolt 9.5 axle quit working after I had filled the axle with Severe Gear from Amsoil. Apparently the friction modifier for posi clutches was too slippery for the carbon clutches in the Gov-Lock. Two changes to axle lube that doesn't have friction modifier, and it seems to be working again.

 

[wirlybird]

At face value the two oils are nearly the same. The 75 might be a little better for severe cold but once the oil is warming up they are the same.
I highly doubt you'd see any improvement in MPG.

 

[df2x4]

As Schurkey mentioned, if you have a G80 carrier in the rear then do not use any gear oil with friction modifier. It will keep the locker from operating correctly if you do.

I use ACDelco 10-4051 80W-90 in both of my '97s with G80s. Works great.

www.amazon.com/gp/aw/d/B007Q10F3S

 

[dryslk]

Just changed my Diff Oil 2 80/90- with MOLY-Slip @ 185000 Miles & I live in Winterpeg Manisnowba

 

[Mike M95 ex cab4x4]

Perhaps a moderator will move this thread to Axles and Brakes. I don't know why you'd post it in Engine Performance.

Do you have G80 Gov-Lock?

My G80 in a 14-bolt 9.5 axle quit working after I had filled the axle with Severe Gear from Amsoil. Apparently the friction modifier for posi clutches was too slippery for the carbon clutches in the Gov-Lock. Two changes to axle lube that doesn't have friction modifier, and it seems to be working again.

Yes I have a G80 according to the codes on my glove box. I think it's 10 bolt 8.5 in, I will double check before ordering the gasket. Thank you for the info.

 

[Schurkey]

Just changed my Diff Oil 2 80/90- with MOLY-Slip @ 185000 Miles & I live in Winterpeg Manisnowba

That would have more meaning if you told us what axle it went into (8.5, 9.5, 10.5, or 11.5 ring gear) and, especially, whether or not you have Gov-Lock.

Yes I have a G80 according to the codes on my glove box.

Then you're in the same boat as I am--unhappy with any of my axle-lube choices, due to GM having vaguely-worded service bulletins and therefore incomplete information.

All I can say is that I crammed a 9.5" Gov-Lock axle under my '88 years ago. Had it apart for inspection, put in axle seals, filled it with Amsoil Severe Gear (75W-90?) and figured I was good-to-go. Within a couple months, the Gov-Lock quit working.

I changed fluid with Valvoline 80W-90 having no friction modifier in it; and it still didn't work. Eventually--not that long ago, just a couple months--I changed an axle shaft, refilled again with the Valvoline with no modifier. Seems like my Gov-Lock is coming back to life.

GM also has bulletins on NOT using synthetic gear lube in the various axles built before a certain date, unless you replace the silicone sealer used in production with actual gaskets. They say that synthetic gear lube destroys the silicone sealer, leading to leaks. I don't put a lot of faith in this--they tested ONE synthetic gear lube and had problems. Doesn't mean EVERY synthetic gear lube will interact with the silicone; and by now the OEM silicone may have been replaced by aftermarket silicone sealer.

I think it's 10 bolt 8.5 in, I will double check before ordering the gasket. Thank you for the info.

The Gov-Lock in the 7.5 and 8.5 versions are commonly referred to as "Gov-Bomb". Be careful. I guess the 9.5 and 10.5 versions have a better reputation.

It would be worth your time to pop the C-clips and slide the axles at least part-way out to inspect the bearing surfaces. Common for the axle shafts to be worn--sometimes severely--at the outer bearing area. Whether you pop in new axle seals is up to you. (Inspect for leaks before pulling the shafts out.)

 

[454cid]

GM also has bulletins on NOT using synthetic gear lube in the various axles built before a certain date, unless you replace the silicone sealer used in production with actual gaskets. They say that synthetic gear lube destroys the silicone sealer, leading to leaks. I don't put a lot of faith in this--they tested ONE synthetic gear lube and had problems. Doesn't mean EVERY synthetic gear lube will interact with the silicone; and by now the OEM silicone may have been replaced by aftermarket silicone sealer.

Do you have a copy of that TSB? I've never seen one saying not to use synthetic in earlier axles. Only the one saying not to use an additive with the locker.