DawgII

Can't get Royal Purple 75W-140 for my 2002 3/4 ton at the moment. Mark from Royal Purple informed last Fri RP will phase out the 85W and keep the 75W.

My local NAPA dealer says this is no difference in performance (all they have is the 85W at the moment), Mark agreed but said NAPA could order the 75W for me if they used the part number he gave me.

What do you long term owners use? BTW the local Dodge dealer says their synthetic blend costs 20 bucks a quart - wow.

Need advice...towing season is right around the corner again.

amartinson

Since you sound like you do a lot of towing, and don't have to deal with Minnesota winters, I'd say go with the 85W-140. A full synthetic will still have a good pour point even with the heavy weight, and will give you the most protection.

DawgII

Basic question: what is the performance difference between the 75W-140 and 85W-140 synthetic blends?

I tow on fairly flat terrain, outside temps vary from 85-110 degrees or so.

amartinson

I looked on Royal Purple and Amsoil's websites and besides slightly higher viscosity, the wear properties between 75 and 85 are very similar. The higher viscosity is important in extreme situations (temperature and load).

RustyJC

We tow a 13,500 lb 5th wheel that puts us on the road at 10,380 lbs GVW (10,500 lbs GVWR) and 21,180 lbs GCW (21,500 lbs GCWR). I switched over to Royal Purple synthetic 85W-140 and have had excellent results. The differential runs quieter and cooler, and no supplemental friction modifier was required for the limited slip.

The 75W and 85W parts of the rating are viscosity ratings at the cold end of the operating spectrum - the 140 indicates the viscosity rating at the hot (i.e., closer to operating temperature) end of the spectrum. Therefore, the only practical difference between the 75W-140 and 85W-140 would be flow at cold conditions. Living in Texas, it makes no difference whatsoever. If I lived in northern Minnesota or Maine, I might be somewhat more concerned.

Rusty

DawgII

Yeah...I'll bet the 85 would be more suited for my winter weather than yours...I take it the 75 is more of a "winter" weight than anything else. The weird thing is the automotive rep I've been corresponding with says the 85 is going to be phased out (I mentioned it earlier).

I have the 75 part number and will gladly post it tomorrow after I get back from work if anyone wants it.

The way my simple crew chief mind understands things....75 is the "thinnest" the lube will be and 140 is the "thickest", right?

In the winter.... I would expect the gear lube to be hovering at the 140 level and the summer 75 ..... am I close?

Junk Yard Dog

Dawg, I'm no oil expert, but, from my understanding, both oils are a 140 weight oil.

The "W" after 75 means winter

Now, theoretically you would think the hotter the oil, the thinner it would be, so either I was "snowballed" at one time or the oil actually does act thinner in the winter.

Hopefully someone with more knowledge will chime in


Rick

amartinson

This is a little technical, but has good info:

What is Viscosity?

"Viscosity is the measure of how thick an oil is. This is the most important property for an engine. An oil with too low a viscosity can shear and loose film strength at high temperatures. An oil with too high a viscosity may not pump to the proper parts at low temperatures and the film may tear at high rpm.

The weights given on oils are arbitrary numbers assigned by the S.A.E. (Society of Automotive Engineers). These numbers correspond to "real" viscosity, as measured by several accepted techniques. These measurements are taken at specific temperatures. Oils that fall into a certain range are designated 5, 10, 20, 30, 40, 50 by the S.A.E. The W means the oil meets specifications for viscosity at 0 F and is therefore suitable for Winter use...

...Multi viscosity oils work like this: Polymers are added to a light base (5W, 10W, 20W), which prevent the oil from thinning as much as it warms up. At cold temperatures the polymers are coiled up and allow the oil to flow as their low numbers indicate. As the oil warms up the polymers begin to unwind into long chains that prevent the oil from thinning as much as it normally would. The result is that at 100 degrees C the oil has thinned only as much as the higher viscosity number indicates. Another way of looking at multi-vis oils is to think of a 20W-50 as a 20 weight oil that will not thin more than a 50 weight would when hot.