my truck has hit the 75,000 mile mark, and i decided that it is time for a total tune up; oil change, diff fluid change, trans drain/refill, etc. but i have a few questions that i'm not able to find answers to, so i am relying on the professionals of this website to give me some advice.

first off, my truck and current statistics:
2001 2500HD 6.0l, reg cab long box (intake, exhaust, plugs and wires)
it weighs 5,000, i don't haul or tow, or drag race (legally)


- i have been running Mobil1 15w-50 in the motor for a couple of years now.
+ i want to switch to Royal Purple, but do i keep the same oil weight that i currently run, 15w-50, or do i go to the GM recommended 5w-30 oil?


secondly; the differential fluid. some of you may remember a previous post from me concerning my whining rear axle. i had changed the fluid at 50,000 miles with Mobil1 75w-90, which might not have been the wisest decision at the time.

- currently run Mobil1 75w-90
+ looking to change fluid with LUCAS diff fluid, but which weight would be best for the rear axle?
- i asked the elderly guy at autozone and he told me that i would get better gas mileage with the 85w-90 fluid, rather than the 85w-140 which excited me, lol. but if i experienced "rear axle whining" while running the Mobil1, should i go for the thicker weight for better protection over more power?

sorry if these questions are beneath you guys, but i figure since i don't have the money for expensive, powerful mods, i want to make my truck last forever so i'm able to mod it later on down the road when i'm finacially capable, haha

 

yea im going to be switching to royal the weight of it i just follow what it says on the cap 5w-30 i think but a 2500HD need a little more weight but one good way to get the old oil out and any sludge drain about 1 qt of motor oil out (doesnt need to be exactly) and put a qt of transmission fluid in it. its a detergent so it cleans like everything and breaks down ur old oil you dont want to mix oils royal with like mobile, etc and after you get the 1 qt of trans fluid in it drive it for 2+ days (dont do too much hot roding the oil gonna be very thin) then drain it and put ur royal in there and new filter of course and its all clean ready to go

 

NEVER put trans. fluid in your motor! The detergent / dispersant package in motor oil is much better suited for the contaminants found in the crankcase.

Mobil 1 and Royal Purple are 100% compatible. Don't worry about the slight amount of Mobil 1 left when you drain the pan - not a problem!

 

um u can put trans in there nothing wrong with it ive done it it always runs better and i heard it from a friend mechanic whos rebuilt like 500+ engines and transmissions currently building his drag car so he knows his **** and if u want royal purple to do its best id put the trans in there i like a clean engine i dont know about u but i wouldnt put something in here that i wasnt sure about or i would have said i wasnt sure. ive already done it works better