I am going to change my differential lubricants in my 1999 F150 4x4 5.4L. Haynes manual says to use 75W-90 GL-5 gear lubricant. I was wondering if it is ok to use the 80/90 from Lucas Oil?

 

IMO, it is imperative that you use a 75-140 gear oil in the rear, and any thing is fine in the front.

IMO, Lucas is nothing more than a gimmick at best.

 

So, I guess I should only use what Ford uses? How about the Royal Purple people? Is there any reason to change to synthetic or "heavy duty" gear oil in my diffs?

 

My 99 F150, called for synthetic, according to the owners manual.

 

So does my 2001. I would suggest checking out the oil forum or rearend forum for more info.
Personally, I would run the recommended synthetic fluid.

 

There are quite a few Ford dealers that use 80W90 gear oil in the rear diffs because of bearing problems. I don't know that it's ever been proven whether the oil caused the problems or if something else changed, like a different manufacturer or something, but it seemed that when Ford went to the 75W140 a lot of bearing problems showed up.
If warranty is buying the oil I use the Ford 75W140, if the customer is paying I usually use 80W90 Pennzoil because our parts department keeps that in stock for people not wanting to spend the big money on Ford oil.

 

I wonder did bearing problems start appearing because with the 75w140 no-one ever changes it?

Ford says if it never gets moisture in it that it is basically good for life.

I have 180,000 KM's on my truck and consistently carry 900lb's in the box so I wonder if I should switch to 80w90? Any suggestions? I changed my 75w140 about 3 months ago.

 

I put lucas in my wife's explorer for the real stiff steering and after 400 miles it is now normal???. could lucas really work???.

 

cgartly the ones I've seen with bearing problems were lower mileage than you have. If you haven't had problems I'd stick with what have.

 

This topic has been beaten to death in the Oil and Lubrication forum.

I'll leave it at that.