2007 ford f150 73,000 5.4 3.55 LS rear end. So about 5,000 miles ago I changed my rear end oil. I used royal purple with the friction modifier already in it. I did not add extra as I read on here that people did not. Plus the website also stated that it was not needed. So about 3000 miles ago I started developing a vibration when turning left or right from a dead stop. After reading I heard it could be the clutches in the Limited slip. I added a 4 oz bottle of modifier and the vibration has gone away. I guess the amount of friction modifier in the royal purple was not enough. Hope it didn't damage anything. Will see what happens in the future.

Has anyone else experienced any issues like that after changing the oil and just using what was in the bottle from a after market company like royal purple or mobile?

Also wondering if I should add more? I know the vibration went away But I believe ford calls for 7-8 ounces.

 

Limited-slip differential requires special hypoid gear lube with the additive or the additive must be added to regular HPL. You didn't hurt anything. It told you it needed help and you responded. However, the differential originally had synthetic HPL for LS diff in it from the factory that is supposed to be 'lifetime'. I wouldn't change it unless it was in severe service, including frequent towing, had been in deep water or it started to become dark and opaque. If you bought it new, the original oil looked okay, the rear had no symptoms and has not been in sustained severe service, I might have left it in.
BUT. As long as you use the right stuff and do the job right, you're probably extending the life of the rear end.

LOL - a lot of words to say 'you done good'.
P.S. - they used to use some kind of whale oil in certain special lubes, and it took them a while to find a substitute after that was stopped.

 

Yeah I do a lot of towing during hunting season. I also tow a trailer around 4,000 lbs in the mountains in north Carolina and Virginia. I had no issues but decided it was time to change it.

Royal purple is synthetic oil and it had friction modifier in it. It said on the site that additional modifier was not needed. I guess my truck needed more.

 

I changed my diff oil last year with Royal Purple and also needed to add a bottle of friction modifier. It has been fine, no problems.