On 8/19/2017 10:48 AM, Bram van den Heuvel wrote:

My one question to you is if the synthetic doesn't thicken as much in the
cold, how does it keep the 75W and if it doesn't thin out in the heat as
much, how does it keep the 90 rating?

Isn't that what the rating is all about?
So doesn't one 75W90 act the same at cold/hot as another 75W90?

That's a function of the VI additives, not the base oil. A higher end oil
is apt to have better and more stable VI additives. This means that after
five years in the transmission, it's apt to remain a 75W90 oil, whereas the
less expensive oil probably won't be.

Now, in extreme cold, the synthetic might well be thinner. Remember that
it's a 75W90 oil meaning that two viscosity measurements are made at two
different temperatures. Once you get outside those temperatures, the
behaviour of the different oils will not be the same. This isn't an issue
here in Virginia but it might be a very significant one in Maine.

These days there are a lot of conventional oils that come very close to the
stability and quality of the synthetics, and in the US most of them can be
sold legally as "synthetic" even though they are not really. This makes
the labelling very problematic unless you look very carefully. The Castrol
Syntec 10W-30 can't be sold legally in Europe as a synthetic oil, but the
Syntec 10W-40 can be. If you look carefully at the type approvals on the
label you can see this.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."