Royal Purple gear oil, which weight?

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Ive been searching for gear oil stuff for a while now, and I think I will be going with Royal Purple.

Now I need to decide which weight to go with. This is for my 2000 Grand Cherokee with Dana 30 (front) and Dana 35 (rear) with 3.55 gearing. I have the Selec-trac 4wd system and the 4.0 engine if that matters.

The manual calls for 75w90 or 75w140. I dont tow anything and rarely carry heavy loads. Our average temps range from 0F to 100F. Will there really be a difference in cold performance since both are a 75Wxx? What about fuel mileage, is there a noticable difference between the two?

Thanks!
 
Both of my trucks call for a Full Synthetic 75W140, so I use the Royal Purple. I tow heavy so I use the Mfg. required oil.

In your case I would use the 75W90. I think the 75W140 is not needed in most cases. You won't lose a noticeable amount of mileage, but with the price of fuel, every 1/2 MPG helps. Also in your winters, the lower grade just makes good sense to me. But I am a fan of using the lightest oil that you can safely run.

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weve been real happy with rp 75w-90 in the rear of my father's 94 toyota previa.

JMH
 
askoop,
I'm in a similar situation as you. My 2002 F-150 calls for 75W-140. But since I don't tow I went with MolaKule's SF HDS-5 75W-90. It's been in for 500 or so miles and is quiet. So was the 75W-140. I have noticed a slight improvement in the gas mileage. Even though at the same time I drained the ATF from the manual tranny and refilled with SF's SynchroGlide. It's more viscous and still my mpg's increased slightly
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. I did notice that I'm able to go up some large hills near my home now with out having to downshift as early as I did previously. Another good benefit from using MolaKules SF fluids
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.

Whimsey
 
I have used RP 75W-90 in two different trucks. The first was a ford F-250 and called for 80W-90 in the differentials. The RP was much smoother, and eeked out 1 more MPG. The truck was used to tow a gooseneck horsetrailer (3-horse) and travel trailer. Four months ago I purchased a 1999 Dodge 2500 (diesel)and it called for Synthetic in front differential 75W-85? and 80W-90 dino in the rear. Manual stated if towing you could go to full Synthetic 75w-140. I have changed both diff's with the RP 75w-90, and have never looked back. I read on a post on this forum that RP 75w-90 Max-Gear is the thickest of that weight of synthetic gear lubes. I have had no problem towing, and will use that weight exclusivly because I don't always tow. And I feel even with towing, that The RP will do great.
 
I LOVE RP's 75w90 max gear. I changed the rear diff fluid in my Tacoma and picked up 1.5-2mpg. You can't beat that with a stick.
 
I am with 59 Vetteman on this one. Use 75W90 no need for 75W140!! Gear lube is one place I do think you see a huge difference in gas millage and cold weather performance in relation to the viscosity. Both weights of MaxGear lube are rated at 100+lbs in the Timken test.
 
I prefer 75-140. Just make sure it is full synthetic. The gas mileage will be about the same as a 80-90 conventional group 2 and it will give much better protection. Especialy if you like to drive hard.
 
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