I'll explain it to you.
The base oil would normally be about a 30 weight.
When you heat it up, it becomes a 40 weight.
40 weight is thicker than 30 weight.
As I understand it, the larger the spread between the w and the at temperature flow, the more prone to sheer it is.
For instance 5W-20 would sheer less than 0W-20.
That would mean that 0W-40 is going to sheer more than 10W-40.
What's most important is what grade of oil do you really need.
I live...
The way I understand it is that 10W-40 is the same as 10w-30 with a bunch more additives in it, to thicken it up when it comes up to temperature. It's these additives that can leave a varnish inside of an engine. And again, the engine is never getting hot enough to thin out that heavier oil.
At...
Here's the deal... Most modern cars are cooled by liquid. They operate in a specific temperature range. The advantage of a heavier oil would be if the engine were running hotter, let's say like in a Porsche air-cooled engine on a summer's day. A thicker oil would be advised in that situation...
We have bought two motorhomes (used) from Cruise America.
They are originally made to a higher standard.
We have a 20' model with a 7.3 in it.
It accelerates uphill.