Gear Oil
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Cross-Country
Joined: Feb 2004
Posts: 80
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From: South Dakota
Gear Oil
I searched around and didn't have much luck finding what I wanted. I'm looking at changing the gear oil in my front & rear differentials. I have a '01 5.4 4x4 expy w/72k miles. I went to the store last night hand had the choice of Royal Purple 75w140 for $10/qt or Motorcraft 75w140 for $16.50/qt. Does anyone know which one is better? I get the feeling I'd be paying for the motorcraft name if I went with that...but if it's really better I'd rather spend the extra $$. On the other hand, motorcraft oil filters & 5w20 oil is priced the same as everything else....
Is Royal Purple better or the same as Mobile 1 gear oils?
Is Royal Purple better or the same as Mobile 1 gear oils?
Laughing Gas
Joined: Jan 2003
Posts: 1,082
Likes: 40
From: Melbourne, FL
Royal Purple is supposed to be some really trick stuff. I can recall one of the car shows (Hot Rod TV, Horsepwer TV, etc.) replaced all the oils/lubes in the powertrain of a vehicle and did before and after dyno runs. They actually gained power with he Royal Purple. Personally, I've never used the stuff though.
As for scottspoerry's concern about the friction modifier, that will depend on whether or not you have a limited slip differential. If you have one, then you need the modifier.
As for scottspoerry's concern about the friction modifier, that will depend on whether or not you have a limited slip differential. If you have one, then you need the modifier.
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Cross-Country
Joined: Feb 2004
Posts: 80
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From: South Dakota
I do have limited slip. On the bottle of the Royal Purple it does say "Friction Modifier all ready added". I guess I can just try it out. If I have problems turning I can just add in some of the ford stuff. I've got a bottle laying around somewhere.
I get the feeling the Royal Purple should be every bit as good as the OEM stuff.
Dan
I get the feeling the Royal Purple should be every bit as good as the OEM stuff.
Dan
Tuned
Joined: Jan 2005
Posts: 261
Likes: 1
Front and rear diffs typically have different viscosity. Pull out your service manual, or the owners manual. Yes, the limited slip rear rear had friction modifier requirement...
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Cross-Country
Joined: Feb 2004
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From: South Dakota
Well, it's all done. The rear fluid looked like new. The front fluid had a milky look to it. NOt sure I like that. Seems like that might mean there was a little moisture in there. Ugh, why don't they put a drain on the rear diff! The front one was so much easier 
I took it for a drive on some pavement. It seems like the friction modifier is ok. I'll try to remember to post a followup here if I have to add more for some reason.
The book didn't specify if the front had synthetic or not. The viscositys were different though. 75w90 up front, 75w140 rear. It took about 1.8 qts for the front and 3.2 for the rear. I topped off the rear (that .2 qts) with my left over 75w90. At $10/qt I didn't want to buy a 4th qt just to use $2 worth of it. Hopefully my attempt to be froogle doesn't tear something up back there!
I took it for a drive on some pavement. It seems like the friction modifier is ok. I'll try to remember to post a followup here if I have to add more for some reason.
The book didn't specify if the front had synthetic or not. The viscositys were different though. 75w90 up front, 75w140 rear. It took about 1.8 qts for the front and 3.2 for the rear. I topped off the rear (that .2 qts) with my left over 75w90. At $10/qt I didn't want to buy a 4th qt just to use $2 worth of it. Hopefully my attempt to be froogle doesn't tear something up back there!
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Tuned
Joined: Jan 2005
Posts: 261
Likes: 1
Originally Posted by vanlohd
Well, it's all done. The rear fluid looked like new. The front fluid had a milky look to it. NOt sure I like that. Seems like that might mean there was a little moisture in there. Ugh, why don't they put a drain on the rear diff! The front one was so much easier
I took it for a drive on some pavement. It seems like the friction modifier is ok. I'll try to remember to post a followup here if I have to add more for some reason.
The book didn't specify if the front had synthetic or not. The viscositys were different though. 75w90 up front, 75w140 rear. It took about 1.8 qts for the front and 3.2 for the rear. I topped off the rear (that .2 qts) with my left over 75w90. At $10/qt I didn't want to buy a 4th qt just to use $2 worth of it. Hopefully my attempt to be froogle doesn't tear something up back there!
I took it for a drive on some pavement. It seems like the friction modifier is ok. I'll try to remember to post a followup here if I have to add more for some reason.
The book didn't specify if the front had synthetic or not. The viscositys were different though. 75w90 up front, 75w140 rear. It took about 1.8 qts for the front and 3.2 for the rear. I topped off the rear (that .2 qts) with my left over 75w90. At $10/qt I didn't want to buy a 4th qt just to use $2 worth of it. Hopefully my attempt to be froogle doesn't tear something up back there!
On using the .2 qt on the rear, you may have saved now, but eventually, you may pay at the pump for it.... May take a long time...
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