I own a 2003 2500HD with the 6.6L diesel. I have now
covered 10,000 miles of which 3,000 miles was towing our
11,000 lb fifth wheel. In January/February we will be
towing the fifth wheel from Canada down to Arizona and
back. Probably about 5000 miles. Do you recommend
changing the rear differential oil and the transmission
fluid over to a synthetic. The fluids are still the
originals. If I have to change the rear differential
oil, does the old oil have to be flushed out, or can
one just drain and then refill with synthetic? What are
your recommendations?

railbuff,


The real is already syn, but should definately be changed. General recommendation is after 5k or 500 miles towing to allow for rear diff breakin. It is very simple....just drain, and refill with 4 qts. The tranny should at a minimum have the spin-off filter replaced by now, and be sure to clean the magnet that is on top of the filter before putting on new filter.


John

all good questions and will be answered soon in my rec maint list


Eric

What oil goes in the rear diff?

SAE 75W-90 Synthetic Axle Lubricant (GM part # 12378261) or equivalent meeting GM spec 9986115.


Or what most do is use a good quality GL5 rated synthetic gear lube ..I run Mobil ...


Mac