I have been noticed on most ford forums that they re using 75w-140.

My Manual book said to use 75ww-90...I remebered that they had some issue with some shudder and other vibrate and they changed it to 75w-140 gear oil for rear end.

The reason why i m asking, please check here. https://www.f150online.com/forums/sh...d.php?t=241184

I was not sure if i should drain it and fill it w/ 75w-140.

I lives in 4 seasons of weather- New York. i dont know if its good idea to put some 75w-140.
Should i go back to regular ford gear oil instead of ROyal purple that i already has it in rearend 75w-90.

Any feedback or suggestion will be great!

Thank you!
YOMAN!

 

75W140 is being recommended as the service lube for axles that originally ran 75W90. In the full frame cars, Ford had a rash of bad axles and axle bearings in '03, the service fix was to replace the axle shafts, bearings and seals and refill the differential with 75W140. If it doubt, go with 75W140. Ford specifies this oil when replacing ring and pinions as well.

 

I also recently changed my rear diff fluid using Royal Purple 75W140. I did NOT add any additional friction modifier, as RP already contains it. Anyway, I have has absolutely no problems (either before or after the change) with my diff clutch packs... Sounds to me like your problem is related the the clutch packs. Maybe they need to be re-packed again. I'd take it into the dealer for service, provided it's still under warranty. And I wouldn't worry about Ford finding out that you're using RP 75W90, after all that's the grade of oil that your diff originally called for.