RP 75w/140 vs. 75w/90 for 8.8 limted slip oil change

[FangStang]

Just purchased 3qts Royal Purple 75w/140 for my 8.8 differential oil change.(According to AM web page, and the recommendation of a thread on this forum) Didn't buy from AM... had gift card from Amazon.
Now other forums recommend 75w/90 for the 2007 GT, and after speaking to an AM rep, they say that it should be the 75w/90. What would the difference be if I ran the 140 instead?? Also.. AM says NO additive required...BUT others say to add it anyway... What should I do???
Car is street driven only...

 

[Newman]

The royal purple should have friction modifier in it already.

 

[hunter1022]

If you are on the stock ford 8.8 then i would stick with what Ford recomends 75W140 and the friction modifier for the Ford Limited Slip. If the RP already has the friction modifier in it you should be good to go if not you will have to add it. As far as 75W90 vs. 75W140 it's mainly a climate thing if your yearly temps are higher the 75W90 would be fine but if you have colder temps and you are going to drive the 75W140 would be better Ford recomends the 75W140 to cover a wide range of temps. I just changed out my differentail and added Lucas 75W140.

 

[cbracoupe]

Does Ford recommend synthetic fluid? I just changed mine out for a Eaton POSI and they said NO SYNTHETIC and 80W90 with modifier(4oz).

 

[FangStang]

If you are on the stock ford 8.8 then i would stick with what Ford recomends 75W140 and the friction modifier for the Ford Limited Slip. If the RP already has the friction modifier in it you should be good to go if not you will have to add it. As far as 75W90 vs. 75W140 it's mainly a climate thing if your yearly temps are higher the 75W90 would be fine but if you have colder temps and you are going to drive the 75W140 would be better Ford recomends the 75W140 to cover a wide range of temps. I just changed out my differentail and added Lucas 75W140.

Colder temps here in Buffalo, NY... but I'll be driving the car in the summer only... And RP 75w/140 already has the FM in it... Thanks!

 

[FangStang]

Does Ford recommend synthetic fluid? I just changed mine out for a Eaton POSI and they said NO SYNTHETIC and 80W90 with modifier(4oz).

My owners manual recommends... "Motorcraft SAE 75w/140 Synthetic Rear Axle Lubricant"... so I guess I'll go with the 75w/140 I already purchased! Thanks for the reply!

 

[mach828]

75w/140 means when the oil temp increases it will be thicker than the 75w/90. If you place a lot of stress on the rear end you want to run the 140, because it provides better protection when towing, racing, high speeds, etc. If you do not drive the car hard and only daily drive it, the 75w/90 will provide less resistance and net you better gas mileage.

What was posted above is incorrect.

 

[cbracoupe]

My owners manual recommends... "Motorcraft SAE 75w/140 Synthetic Rear Axle Lubricant"... so I guess I'll go with the 75w/140 I already purchased! Thanks for the reply!

There you go.

 

[hunter1022]

I have the Eaton TRUE TRACK and Eaton said no Synthetics and FM not required and I thought that went for the Eaton Posi as well but it's all in what there tech told you .

 

[cbracoupe]

I have the Eaton TRUE TRACK and Eaton said no Synthetics and FM not required and I thought that went for the Eaton Posi as well but it's all in what there tech told you .

Your TrueTrac is a gear Diff. The POSI is a clutch diff.

 

[AMWill]

Just purchased 3qts Royal Purple 75w/140 for my 8.8 differential oil change.(According to AM web page, and the recommendation of a thread on this forum) Didn't buy from AM... had gift card from Amazon.
Now other forums recommend 75w/90 for the 2007 GT, and after speaking to an AM rep, they say that it should be the 75w/90. What would the difference be if I ran the 140 instead?? Also.. AM says NO additive required...BUT others say to add it anyway... What should I do???
Car is street driven only...

Hey Fang,

I noticed that I shot you a reply in another Forum already but I just wanted to reiterate that you will have no issues.

The 75w140 is completely fine and I run it personally!

Along with that the Royal purple already has friction modifier included so you would not need any additives!

Hope this helps and let me know if you have any other questions.

-Will

 

[hunter1022]

Dicked up that post the 75W90 and 75W140 work very similar at lower temps the 140 will offer better protection at higher temps when the gears get hot the 90 better mileage from what I read Ford recommended 75w140 for the wide range of temps and conditions the cars would be sold in, sorry if I confused. I was aware that the Eaton Posi has carbon fiber clutch disks I had issues with the ford traction lock and was not going to get anything with clutch disks again thus the True Track and when talking to Eaton I thought they said they did not recommend synthetics for there products but I may have misunderstood as I had asked them questions about 3 different differentials, again sorry for the confusion.

 

[cbracoupe]

Dicked up that post the 75W90 and 75W140 work very similar at lower temps the 140 will offer better protection at higher temps when the gears get hot the 90 better mileage from what I read Ford recommended 75w140 for the wide range of temps and conditions the cars would be sold in, sorry if I confused. I was aware that the Eaton Posi has carbon fiber clutch disks I had issues with the ford traction lock and was not going to get anything with clutch disks again thus the True Track and when talking to Eaton I thought they said they did not recommend synthetics for there products but I may have misunderstood as I had asked them questions about 3 different differentials, again sorry for the confusion.

Correct, the directions said no synthetic. However, I'm pretty sure the OP still has the Ford Traction Lok. And the Ford Trac Lok is not quit the same as the Eaton Posi It's been debated many times. IMO, the POSI is a better diff for DRAG use. Especially when upgraded with heavier springs etc. The clutch diffs mitigate shock much better. Gear diff in street/road racing setups. But again, this has been discussed many times and some ppl have luck in drag applications with the gear diffs. Lots of factors, tires, track, transmission etc. Sorry OP to get off topic