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Discussion Starter · #1 ·
I have read many times on this and other forums that 75w-140 should be used if you are towing heavy loads. I have a 2003 1-Ton and when monitoring the temp in the rear end it never goes over 190 under normal conditions but when pulling a 7 mile long grade at 7% the temp rose to 265 deg.This has happened several times. I am looking to lower these temps. first I purchased a Mag-Hytec cover to increase the amount of oil but do not really know which weight oil to use. Nowhere in my manual can I find any reference to 75w-140. Is it possible the heavier oil will cause higher temps?

Rich Miller (Poorloser)

448 Posts

I think the reference to 75w-140 is in the service manual. I think it was for my 95, I don't have a service manual for My 01 yet. I run the 140 oil myself. It might be overkill, but I'm pretty sure it would make the rear end run cooler under extreme usage not hotter.

448 Posts
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Diesel Enthusiast

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<TD class=text vAlign=top background="" bgColor=#fefefc>G80

from Eaton:
"GM introduced the new body style in 1998. At the same time they upgraded
the differential and changed to a synthetic lube. The G80 is specifically
designed to give ultimate performance with this lube, and the lube contains
special additives that help reduce slip stick that occurs between the
clutches. the GM synthetic lube typically runs about 100+ degrees cooler
than mineral base lube, which is better if you do allot of towing or heavy
hauling. I would recommend that you stay with the GM synthetic, not Mobil 1.
If you don't tow or haul you could use the pre 1998 mineral lube without
degrading performance of the differential. this would be a cheaper option
but require more lube changes. "
"Axle tolerances of the 1/2 ton trucks are very tight. The recommended
procedure was to change lube after 500 miles of heavy towing, which would is
considered a break in period. This breaking period generates high heat
conditions in the axle which was found to degrade the synthetic beyond the
point of mineral lube. After the 500 mile lube change, the axle is broken
in and does not generate the high heat.
If you use GM synthetic after the
break-in period, it can be considered "lube for life", although some say to
change it at 40,000 - 50,000 mile intervals."
"If you don't tow, don't worry about the oil change as it a waste of money."

"If your truck is pre 1998 or the old style, it comes with mineral lube. IN
that case you need to stay with mineral because the seals are designed to
work with it. If you changed from mineral to synthetic you'll cause your
seals to swell and eventually leak. It's ok to go from synthetic to
mineral because you won't cause swelling. many of your stop leak
chemicals effect seals in this manner, resulting in a short term fix but a
long term problem."

And this,
"Back in November, I received the following:
"From Mr. Ralph Holmquist of Eaton, the maker of the locking differential:
"The maintenance schedule for the rear axle was developed by American Axle &
Manufacturing and GM truck based on multiple tests. The Eaton locker does
not require additional maintenance nor does it add heat to the lube. The
lube will darken due to the carbon wear on the clutch surfaces
, much the
same as a disc brake pad & rotor. This does not damage axle components such
as seals or bearings. However, a new axle can produce excessive
temperatures (plus 350 degrees F) due to the ring & pinion breaking in that
will break the lube additives down. Avoid high loads, trailer towing and
high speed extended driving during the initial break in of the vehicle.
After the break in period axle temps will level at a much lower figure.
Lube changes are a good idea because the additives are replenished and
contaminates such as casting sand are eliminated. The axle is filled at the
factory with a synthetic 75w90 GL5 rating made by Texaco under part # 2276.
The GM service # is 12378261. This is the only lube we have done extensive
testing with to insure locker compatibility. The only negative to using one
of these other lubes is an increased potential for clutch chatter. This
really doesn't hurt anything and can be corrected by changing the lube. I
noticed in the latest GM owners manual the term "or equivalent" when
referring to the lube specification. Look for a GL5 rating on the bottle to
make sure the ring &a
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