Gear Oil...Royal Purple or Amsoil??

I only hear good things about Royal Purple. I think it only takes 2 or 3 quarts. I could be VERY wrong.

It takes 75W-140 synth with 4ozs of friction modifier added. Fill till it runs out the fill hole. More info at www.bullittarchive.com

Royal purple has my vote
I use royal purple in everything

I think I used 70 weight

Royal Purple, and about 2 to 2.5 quarts

So Royal Purple is better!

Its the f'n differential. Just use what they sell at the Ford parts counter. Definitely at least get the friction modifier from Ford...

75-140 synthetic... there is a tag right on the differetial that says this. Old schoolers will say 80-90 dino gear lube, ignore them...

Someone is going to throw the flag up for what I'm about to say, but I'll say it anyway.

When I had my rearend done last year, the guy who rebuilt it for me (30+ year in Ford's axle plant garage) told me NOT to run synthetic oil in it right away. He said to use Motorcraft (or equivelant) oil for the first few thousand miles and then go ahead and pull the cover off and clean it out and run synthetic. He said to definately switch to synthetic oil though.

I was also told by a buddy of mine who is a Service Write-Up guy(and assistant service manager) at a Ford dealer to stay away from Amsoil gear lube. He said they've noticed that it breaks down and turns chalky or faomy. He suggested I go with Royal Purple insted.

Go ahead away!

I put Royal Purple 75-90. YOU DON"T NEED THE MODIFER WITH ROYAL PURPLE. It's already in there. The factory stuff , I believe, is synthetic anyways.

It will take 2 quarts with the factory cover, 3 quarts with the support girdle cover.

There is a tag right on the differential bolts that says "75-140 synth only". I would suggest that. Get the lube and the friction modifier from the Ford Parts counter. End of story.

Kilgore Trout said:

There is a tag right on the differential bolts that says "75-140 synth only". I would suggest that. Get the lube and the friction modifier from the Ford Parts counter. End of story.

Click to expand...

Okay, I read this thread and did some looking in my Amsoil application guide. I have a 99 GT and have been running the Series 2000 75W-90 for 4+ years now with no issues. Last fall Amsoil improved their Gear Lube line and released new formulas to address the harder abuse, and heat many rear ends now go through, plus the lesser capacities they're being made with. This is their Severe Gear Gear Lube and comes in 75W-90 and 75W-140. I've never heard of Amsoil foaming or breaking down as described. In fact I've heard just the opposite. One Dealer I have worked with purchased a Powerstroke when they first came out and ran it hard, between modifications, hauling, towing, etc. After about 15K he sent a sample of Ford's "lifetime" synthetic in and found it had lost it's viscocity and also he noted the discoloration of his gears from heat. This fluid was breaking down. No such problems since putting in the Amsoil. Also one thing to note is that the synthetic Ford is using is a Group III petroleum base fluid. This fluid does fine until the additives that keep it stable wear out, then it plummets in performance. The PAO based Gear Lubes stay more stable and go through a gradual decline over time.

I've spoken to Circle Track racers who used both RP and Amsoil and liked Amsoil better. The RP rep in the latest issue of MM&FF commented that all synthetic is made from crude, which is the case for Group III synthetic only, so I can only assume RP is now using this type of oil. I don't know if they had started using this for their other fluids yet, but before putting it into my tranny, I'd definately want to look into it.

As for which viscocity, my 99 GT calls for a 80W-90 according to my owners manual. I looked up the Amsoil recommendations and it too for that year called for that viscocity and had their fluid recommendations. It shows two diffs, one being the standard, the other the Controlled Slip, both using the same type of fluid. I looked up the 2002 that camarokiller has and there were two different different specs. The standard diff still calls for the 80W-90, so our Severe Gear 75W-90 would work fine. But if you have a Controlled Slip Diff. w/ 8.8 ring gear, this is calling for a 75W-140. This must be what stangfreak has. Again Amsoil's Severe Gear 75W-140 would work here. As for any sort of additive, Amsoil doesn't recommend it with their fluid, but should you have any noise, they do have their Slip-Lock Differential Additive.

BTW, I didn't see a tag on my diff. unless it was removed at some point by the previous owner or when my fluid was first changed.

Hope this helps.

no tag on my diff either