Is it safe to use 75w90 in place of 75w140 in a 99 F150 rear end?
I plan on using synthetic and have heard (on this forum) of people
using 75W90 with good results? I can and will use the recommended
75W140 at dealer price if that is what most agree is best??
Just want opinions???

 

If it is under warranty, you should use what Ford tells you to. Out of warranty a good synthetic should work fine The 75W90 syn has such a wide temperature range that it overlaps the 140W. Synthetic oils are thin but have good temperature tolerance.

If you have a limited slip, you may need some limited slip additive.

This is just my opinion based on reading and using synthetics for over 20 years.

Good Luck,

Jim Henderson

 

 

I know this has been discussed, but I am currently changing my rear differential fluid and I got some Mobil 1 75w-90. However, I hear that with heavy towing, I should be using 75w-140 instead. I tow about 9,000 lbs. This is confusing.

 

Another question - what was the factory fill back in 1989? I want to use what they used.

 

With synthetic, you should be fine with 75W90. I use Mobil 1 75W90(I have also use Valvoline syn and been satisfied) in my F250 supercab towing an 8,000# trailer. I tow thru sub freezing to desert heat including steep long hill climbs. I would estimate my total combined weight to be a bit under 16,000 with full tanks and junk in the trailer.

I would guess I have towed for about 20-30K miles with no problems and last inspection looked like new with no unusual bits and pieces in the drain pan.

You could go with a 75W140 and if it is syn you should be fine. With dino, it might get thick at cold sub freezing temps. I think the only consideration with syn now a days is whether you worry about the tiny gas mileage differences between 75W90 and 75W140. Unless you tow a big 5th wheeler all the time in Saudi Arabia, I think the 75W90 will do good. 140 won't hurt though, unless you live in Siberia.

Only thing I would wonder with a 75W140 is whether the gear lubes have the same problem motor oils had with wide range viscosities. The wider the range of VI improvers in motor oil, the more likelyhood of the polymers (VI Improvers) failing and sludging things up. But since we are talking gear lube, I have no clue and haven't read about any problems.

Best advice is to follow what the manual says to use, taking into account your application and temperature ranges. The manufacturers spend a lot of time and money to figure this one out for you.

Just my opinion,

Jim Henderson

 

I recently changed the front and rear diff. fluids in my 2000 EB 5.4L Expy. I went with 75w90 Mobil 1. I have used this in other diff's before and have no sludge build up. When the front drain and fill plugs came off, they had a thick sliver-grey sludge all over. The rear fill plug was the same. I replaced the rear ABS sensor as well, it was giving a trouble code, and it had the same silver-grey sludge on it. The fluid itself looked like the old Arco graphite motor oil from the 80's (black with silver irridescence).

Is the sludge related to the ltd. slip additive, or is it because ford doesn't recommend changing the fluid and it is wearing out and gunking up because of all the vis. index improvers that are in the heavier 80-140w gear oil? Any thoughts?
The Bonz

 

When I changed my gears, I put Mobil 1 75w-90 in the front differential, and Amsoil 75W-140 in the rear. The tag on my rear differential specifically stated use only synthetic 75w-140. Since Mobil 1 did not make that wgt, I used the Amsoil. I would have preferred to keep it all Mobil 1 though.

 

I have heard Mobil 1 does make the heavier oil, it just doesn't get stocked at many stores.

Anyone have an idea as to the silver/grey sludge on the diff. drain/fill plugs and abs sensor? I have gone w/Mobil 1 75w90 in the rear after hearing many opinions and with my past experience.
Think of this, if engines that can are spec'd for 5w30 can retroactively use 5w20 or 0w20 oil, I can't see a problem with lighter oil in the rear end, especially with no towing in the plans.
There have been more than enough post on this site of users going with the lighter rear end oil and I have yet to hear of one who did it and it broke. Lots of naysayers for sure, just none who have tried it yet.
Bonz.
Bonz