After reading horror stories about GM not filling the differentials to save oil, I asked my GM Service Rep. to check all fluid levels at the last service in May. I had asked for a drain and refill in 2006 when the truck had 600 miles on it, which he refused, saying that it wasn't needed until 75,000 miles unless I tow.

While at the Amsoil store today I picked up a case of Extreme Gear 75W-90 and crawled under it for my own look around. Yes indeed, the rear differential had a little less than 2 quarts of blackened oil, heavily contaminated with metal particles, while the plug magnet looked like somebody dropped a fifty pound magnet in a steel wool factory. That crud has been circulating for 15,000 miles.

Looks like nobody checked the differential fluid level, or GM considers 1/2 full all that's required. The front was cleaner, but all the fluid drained from both units topped up a single one gallon plastic jug. Both were refilled with the Amsoil to the proper level as noted in the manual.

Folks, I have differentials that are 20 to 30+ years old with hundred of thousands of miles of service, including towing, with not one leak or replaced axle bearing. Fluid was always changed at the first 500 to 600 miles, and every year before towing season or at 25,000 miles. Fortunately, there's no damage, but you just can't trust or believe anyone except yourself. Changing the fluid is real easy, about a 3 on my Crippleometer, so even gimps like me with an air wrench and 3/8" socket set can do it on two Diet Cokes and a Vicoden.

After reading horror stories about GM not filling the differentials to save oil, I asked my GM Service Rep. to check all fluid levels at the last service in May. I had asked for a drain and refill in 2006 when the truck had 600 miles on it, which he refused, saying that it wasn't needed until 75,000 miles unless I tow.

While at the Amsoil store today I picked up a case of Extreme Gear 75W-90 and crawled under it for my own look around. Yes indeed, the rear differential had a little less than 2 quarts of blackened oil, heavily contaminated with metal particles, while the plug magnet looked like somebody dropped a fifty pound magnet in a steel wool factory. That crud has been circulating for 15,000 miles.

Looks like nobody checked the differential fluid level, or GM considers 1/2 full all that's required. The front was cleaner, but all the fluid drained from both units topped up a single one gallon plastic jug. Both were refilled with the Amsoil to the proper level as noted in the manual.

Folks, I have differentials that are 20 to 30+ years old with hundred of thousands of miles of service, including towing, with not one leak or replaced axle bearing. Fluid was always changed at the first 500 to 600 miles, and every year before towing season or at 25,000 miles. Fortunately, there's no damage, but you just can't trust or believe anyone except yourself. Changing the fluid is real easy, about a 3 on my Crippleometer, so even gimps like me with an air wrench and 3/8" socket set can do it on two Diet Cokes and a Vicoden.

I agree. I've been a GM Tech for a long time and have seen some nasty rear diffs.

So what is a good change cycle for both the fronts and the rears? I am guess they do not require the same maintenance?

I am just getting ready to change my rear.

Changing the fluid is real easy, about a 3 on my Crippleometer, so even gimps like me with an air wrench and 3/8" socket set can do it on two Diet Cokes and a Vicoden.

:thumb:

Moved to Maintenance and Fluids!

Yep, that was an easy change, especially the rear. The only difficult part was removing the skid plate, glad I had the air ratchet (cheapo HorrorFrieght model) for that one.

drained and measured mine . 3 quarts ! Is the other quart in the wheel bearing ?

drained and measured mine . 3 quarts ! Is the other quart in the wheel bearing ?

Maybe, but if it's never been changed since the factory - that could be all that was in there.

I had to squeeze the truck in the garage so it was dead level and wait for everything to drain out. Traveled on the freeway to warm everything up prior to draining. Also mine is the Eaton G-80 locking diff.