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RP 75W-90 Differential Oil Gear Hum/Groan

s550v6

Mike
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Hello everyone, I have been replacing my stock fluids over the past few weeks in preparation for a 10k mile road trip this summer. Since Midas changed out my rear differential fluid two weeks ago I have been having a groaning or humming noise occurring during on-thottle sharp turns, where the car was previously silent.

I have been in contact with RP, whos customer service is awesome, and they told me to do some slow, tight figure 8s in a parking lot which I did (30 total). This oil also has 500miles on it already and is silent when going straight or turning without any throttle. I just ordered some Ford XL-3 fiction modifier and am going to try and add it at 1oz increments until the noise goes away.

My reason for posting is, has anyone else experienced this groaning humming noise since changing their rear gear oil? I have the stock 3.15 rear differential as well. On more than one occasion I have also had the inner wheel skip slightly during turns with a generous amount of throttle, while the outer wheel stays totally hooked with the pavement. Is this a bigger issue than gear oil?
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xXANCHORMONXx

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I wouldn't put that purple crap in a pinto.

I've used Amsoil and Motul without modifier in multiple s550s with zero issues.
 

15RRGT

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The XL3 will cure the noise. The inner wheel slip while turning on the throttle is normal in most of these cars.
 

abmobil

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I wouldn't put that purple crap in a pinto.

I've used Amsoil and Motul without modifier in multiple s550s with zero issues.
I was wanting to use the Amsoil Severe Gear 75w90 in mine once I hit 5,000 miles. Is that the one you used?
 

OneFordGT

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Mine makes the same sound when I leave my house in the morning while backing out and turning. I didn't notice this until I switched the lube out with BG. BG claims it has friction modifier in it already, think I should add some friction modifier?
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