im kinda new to this forum but not to truck i didnt see anything that was useful in searching so ill ask.

first thing i had my truck lowered by cutting off the spring perch with a plasma cutter off the top of the axle and welding to the bottom to make my own flip kit, i got this done for free couldnt pass it up. this was back in 08 everything has been fixed im just bringing yall up to speed. long story short, i ended up having a small pinhole leak that the plasma cutting shot thru the top of the pass side axle, oil had been leaking there over time i put who know what kind of gear oil, after about 8 weeks, i got with the guy who lowered it and we fixed the leak and cleaned everything out and put new 75w90 full syn back in it.

which brings me to my question, i have been running the 7590 since then and never had a problem, of course i added to additeive for the ls. however he lately i had a problem not bad dont think anyone would notice it but me, and i have no ideal how to explan it. but my main question is i know it says to run 75w140 syn that oil is $$$ but by me running the 7590 would that cause any problems with anything?

 

I would run what ford suggests if it was me.