I have a Currie-built Ford (gasp) 9 in. rear end in my Camaro, currently out of the car and dissassembled for cleaning, new bearings, seals, etc. I had planned on using Royal Purple lube, but the Currie folks are very definite about recommending non-synthetic 85-140 oil. They say that there is a lot of gear contact with a Ford 9 in., and it is therefore better to have an oil with a thick molecular structure for "cushioning", unlike synthetic which is "thin" and washes away faster, and also tends to leak out the seals more. They say that with a synthetic there is less drag and therefore better performance, so it is good for racing, but not for everyday use.

On the other hand, Royal Purple is rated at 75-140 viscosity, which is about the same as 85-140, so I don't get how it is "thin". If you read the claims on the bottle, it claims to promote maximum gear life, has cushioning molecules for shock load protection, meets GL-4 and 5 requirements (whatever that means), reduces friction and lowers temperatures, etc.

Has anyone out there ever heard of problems with synthetics? I wouldn't think Currie would have any "bias", so I am inclined to believe them, but on the other hand, the Royal Purple (which BTW I have already bought) claims to be better.

If Currie says you need to stay away from synthetics, I'd follow their advice, but my personal experience is different.
All my differentials run Amsoil synthetic and have for the past 25 years. I have been using Amsoil exclusively for that long, and all the diffs I've used in have never required any kind of repairs. My Camaro 12 bolt is running strong at 37 years of age with absolutely no repairs yet. My daily driver is at 26 years and doing just as well with a 8.5" 10 bolt.
A few years ago, I had an 85 Caprice with the crappy 7.5" rearend. Early on, I switched it to Amsoil, and even the nasty shifts of the 700R4 didn't hurt that weak rearend in 160000 miles until the car got wrecked. I'm hooked on Amsoil synthetic lube no matter what I hear now.

I have run synthetic in my Currie rear since new, or around 5 years. It has been either Valvoline or Mobil 1. Neither has given me any grief. I change the fluid every year.

Thanks for the quick replies!

The whole thing sounds a little fishy to me--i.e. if synthetics weren't any good, you would think the people who make them would go out of business. I wrote to Royal Purple too. Of course, they have a built in bias, and the Currie people do not, but I still want to get their input.

Rick, your experience is telling, although I am not committed to changing fluid every year. (BTW, GO BADGERS!) :)

Maybe I will "live dangerously" and give it a try.

Just my two cents worth and I'm sure it'll be refuted. But I put Mobil One in two rear ends, ruined both. Started leaking pretty quick and when I pulled the cover to replace seals the oil was full of metal flake. One rear had the spider gears just peeling off metal, the other had a carrier bearing shot, and both were showing wear on the R&P. Both were perfect when I put the lube in, which is why I spent the extra money in the first place. I'm sure many have used the synthetics with good results, but mine sure weren't.

Thanks for the quick replies!

The whole thing sounds a little fishy to me--i.e. if synthetics weren't any good, you would think the people who make them would go out of business. I wrote to Royal Purple too. Of course, they have a built in bias, and the Currie people do not, but I still want to get their input.

Rick, your experience is telling, although I am not committed to changing fluid every year. (BTW, GO BADGERS!) :)

Maybe I will "live dangerously" and give it a try.

I change the fluid simply because "it makes me feel good". I have done a lot of Power Tour trips over the years with this rear under pretty harsh, very hot conditions. I'm sure there is some sort of logic in the Currie techs thought process but without talking to them I can't say exactly what it is. If I remember when I'm down at the PRI show in Dec I'll ask them about it.

One thing I have confirmed with my Trac Loc diff is even with the synthetic I still need to add a bottle to a bottle and a half of LSD fluid to keep it from chattering.

Yup, go Badgers, although I don't think they will go too far this year.

Royal Purple in my rear end?

Thought this topic was about Barney in your poop chute.....:confused:


Well, if its for warranty purposes, sticl to what they say....but if its OUT of warranty.....do whatever you want....

+1 to that!
you really cant go wrong with RP. I use it in my camaro and after the first 500 miles I changed the diff fluid over to RP in my 06 Silverado SS

I thought I was in the SP section for some reason.

OK, I may well regret asking, but what is the "SP" section?

Interesting (sort of) development... I wrote to Royal Purple, and in an effort to be “PC”, I did NOT name my “reliable source” (Currie Enterprises), but I have to tell you that they knew right away who I had gotten my info from. So, this must be a "thing" with Currie that has come up before, and the Purple people know about it. I don’t understand the reasoning for Currie's assertions—I don’t think it is anything against Royal Purple per se, just a bias against synthetics in general. And…I wouldn't think that Currie has anything to gain by recommending conventional oils over synthetic, so I have to think it is their sincere recommendation. However, their opinions may not be “evidence based”, i.e. it may be a result of one or a few anecdotal experiences rather than true scientific research or reasoning.

Interesting too that Royal Purple has a 900 HP panel truck with a Currie rear end, and you KNOW what kind of lube they use. There is a lot of stuff on the RP website about how good it is, and all the shows on Spike TV use it in their projects, so how bad could it be? :)

My rear end is about 17 years old, so I am not worried about warranty anymore. The purple stuff is going in, if it burns up, I'll let you all know (but I doubt if it will).

Dan

you will be just fine. Just use the lube straight, no additves needed.