Best rear end gear oil to use (75w90 or 75w140) - Ford Mustang Forum

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post #1 of 10 (permalink) Old 12-22-2011 Thread Starter
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Best rear end gear oil to use (75w90 or 75w140)

Hey guys, I have a 2006 Mustang GT with manual transmission. I had rear end work done a few days ago and ever since, it's groaning in the corners. I'm pretty sure they put 75w90 in the rear with one bottle of friction modifier. Doesn't sound like that was the best choice. Shouldn't I really be running something like Royal Purple 75w140 with at least a bottle of friction modifier? Especially since I now have 3:73 gears. I also don't know if my car has the updated clutch pack from Ford or not but I never had a problem with noise in the corners before (nearly 80,000 miles on the car).

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post #2 of 10 (permalink) Old 12-22-2011
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I believe the recommendation is 75w90 with friction modifier

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post #3 of 10 (permalink) Old 12-22-2011
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75w90 is wrong the 140 is right and every tech at ford i talked to uses 2 bottles of friction modifier but using one or two is really up to you

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post #4 of 10 (permalink) Old 12-22-2011
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post #5 of 10 (permalink) Old 12-22-2011
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You did not specify the work you have done but I would comment that this is the issue and not the oil.

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post #6 of 10 (permalink) Old 12-22-2011
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says 75-90+friction modifier in the book. hasnt given me any problems.
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post #7 of 10 (permalink) Old 12-22-2011 Thread Starter
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Well, the work I had done was the pinion bearing replaced and the Ford Racing 3:73 gears installed. Right now it's using 75w90 with one bottle of friction modifier. I don't know what that 75w90 is though at this point. Haven't been able to talk to the tech yet today. Problem I'm having is that it groans in the corners now. Never did that before (80,000 miles on it). Short of him opening it back up and going through everything again I'm wondering if its really a fluid issue. Sounds perfect when the rear end is cold but as soon as the rear end warms up I get the groaning and even some gear chatter.
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post #8 of 10 (permalink) Old 12-22-2011
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My 2007 owners guide says 4.25 pints of SAE 75W-140 synthetic.

When I changed the diff cover, I used Royal Purple 75-140. If anything, it seems quieter than stock.

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post #9 of 10 (permalink) Old 12-22-2011
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I would use the 75W140 (I like Amsoil) like Ford calls for (synthetic) along with a bottle of Ford friction modifier.

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Rehagen racing uses 75w90 Royal Purple or Mobil1 in their race cars. And they get beat on so hard they need coolers.

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