the input seal is leaking on my 06 f150 the manual calls for 75w140 and a friction modifier was wondering if any body has heard that 75w90 with the friction modifier be used as it is muchh cheaper and working in a truck shop i can get it very cheap the 140 is about $20 per litre thanks in advance for any input

 

No idea. Why not just change the seal? If things are getting out the seal, surely they are getting in too. How bad is it leaking?

 

its leaking bad enough that its making a mess of my diff and it's approx 1 L low and was gonna do seal and diff cover reseal at the same time just hurts a little bit to pay $80 for 4 L of oil i did notice on really cold days this winter that the oi 75 140 had quite a bit of resistance in it for approx 3 to 4 blocks on the morning

 

I know what you mean about the $$$. If it were me though, I'd bite the bullet, do the pinion seal AND the axle seals, drain and clean out the housing, put a good seal on the differential cover, and fill it with the best gear oil I could get my hands on. If you're doing the pinion seal anyway, the axle seals aren't all that much work, and cleaning out the housing (don't use solvents, just wipe it all out) is a no brainer if you have the cover off.

You are going to want to use the oil that Ford specified, as well as the correct friction modifier. You have a clutch-type L/S (limited slip) differential, and the lubrication properties are critical to it's correct operation. Putting the wrong stuff in there will either:

1) Cause the L/S to act like an open differential and give you less traction.
2) Cause the L/S to grip too much and wear out prematurely, then acting like an open differential and giving you less traction.

You also might check to make sure your axle breather tube is clear from any obstruction. It that has gotten plugged, the oil may be burping past the seals when the axle heats up and the air inside expands.

 

the breathers are clear and i good idea on the axle seals though gotta pull diff cover to drain all oil so was gonna reseal with what we use for highway tractor diffs its really good stuff and i already bought the ford recomended friction modifier just not the oil yet