Topic: Gear Oil Question

[blackdiamond]

Yesterday I changed my rear differential fluid from Royal Purple 75W-140 (full synthetic) to Valvoline 85W-140 (petrolium oil) and I noticed then when I drained the old warm oil it flowed into the pan easily, but when I poured a drop of the ambient temperature Valvoline on the pan it hardly moved at all.

I thought that multigrade oil got thicker as it got warmer?  Is there really that much difference between synthetic and conventional?  The RP was finding leaks so I went back to the old school to slow things down, hopefully.

[mr4x42u]

From what I've seen all oils and grese thins out with heat..I think what you see is just the differnce between the syn and pet oil's..

[blackdiamond [OP]]

Isn't the whole point of multigrad oils to be thin at startup and then thicken with heat?

[mr4x42u]

I'm not quite sure how to explain it so it makes sence...the oil does get thinner,,however the point to where it brakes down gets better in multi weight..not really thicker as if you were to compare water and the thickness of syurp..

[kyle_22r]

i think with multigrades, it's not the thickness, but the lubricating properties that give it its designation.  for example, at 0*, a 10w30 oil will lubricate like a 10 but lube like a 30 at operating temp.  for the most part the thickness seems to be in between.

and another interesting thing to think about:  the wider the viscosity gap, the more polymer viscosity modifiers are in the oil and less actual oil.  because of this, *in theory* at operating temp, straight 30 would be better than 10w30, which would be better than 5w30.  however you have to account for dry cold starts which the 10w30 and 5w30 would give much better initial lubrication.  it's all a bunch of trade-offs, that's why there's so many different oil choices

[blackdiamond [OP]]

I just attended a short "class" at howstuffworks.com and here is what I found:

"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."

For Example lets us 10W-30 motor oil.  If we has a single weight 10 motor oil it would thin to say 5 at 200 degrees, and if we had a single weight 30 motor oil it might thin to say 20 at 200 degrees.  Then the multigrade oil 10W-30 would be a 10 at "cold" temperature, but would only thin to 20 at 200 degrees.