Can someone please explain oils to me?

[Hightower]

It's nearly service time again and I have a query.

Currently in my gearbox, t/box and axles I currently use a 75w90 fully synthetic oil.  Never had any issues with it, other than it's quite pricey (and that doesn't really matter if the stuff is good).

However, when you read about such oil as Difflocks Evolution 1, it states that it has the perfect viscosity, at 5w20, for my type of gearbox.

My question is what, in terms that I can understand, is the real difference between 75w90 and 5w20, and which one should be better for my car?

Any light on the subject is most appreciated.

[davidlandy]

I know that the numbers refer to the viscosity of the oil when cold/hot

thats it!

[Xtremeteam]

TBH you should be using MTF94 or dexron 3 atf in the gearbox as i doubt that the 75w90 is right for the gearbox considering its designed to run on the MTf stuff

But thats just my experiance working on them day to day

[Defender]

75W90 in the transfer box and diffs. Difflock's Evolution 1 in the main gearbox.
Don't whatever you do, put 75W90 or similar in the main gearbox as it's too thick to work with the oil pump in the main box.

[beast5680]

does this bit of text help

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.