Hey guys.. just browsing through the oem manual.
for heavy towing duties, it states to run 75w140..
does anyone run this? if i put it in my rear end will i screw things up?

and what is considered heavy towing?
i tow a 6-10k lbs bi monthly. with the remainder of the time the truck is used for parts runs around town and go 150 miles a day to work..

 

My work truck is a 2006 F550, and comes in at about 18,000 lbs in what I classify as empty. It has a 9 foot service body, crane, air compressor, blah blah blah. We run 75-140 in the rear end all the time, and have never had any problems. We have 4 of these style of truck, ranging for 2005 to 2007, all weighting about the same, and they perform the same duties.

With the weight your towing, I don't think it is necessary, but won't hurt anything putting it in.

 

I have a small 99' Cherokee and the manual calls for 75W 140 when towing. So I run 75 W 140 in the rear as demanded. It's doing fine.

Heavy towing is easy to describe.
The extra load stresses the gearing, because the extra load has to be acccelerated too and is added to the gross vehicle weight. The load is put on the gearing of course.
A thinner oil like the75W 90 is pushed aside very easy. A thicker oil compensates heavy loads under heat. look at it like greased bearings. the oil takes the load variations, minimizes friction and takes away the heat.

The second number is the viscosity of the gear oil at 212°. That means 75W 140 has a viscosity of 140 at 212° compared with a viscosity of 90 at 212°.

 

awesome.. thanks for easing my mind guys..

 

It won't hurt anything. The only thing it could do is decrease mileage a bit, but even if it does, it won't be much.