At 01:32 PM 9/13/2007, David Kao wrote...
>I have long suspected why the Vanagon community strongly recommend
>20w-50 oil and everybody buys that. I do not intend to start a war
>on oil weight discussion. But 20W-50 dino oil means 20 weight oil
>in winter (50 in hot summer).

No, 20W is the viscosity rating at -10C, 50 is the viscosity rating at
100C (approximate engine operating temperature). Unless your engine
doesn't heat up in the winter (i.e. the oil stays _cold_), the
viscosity at operating temperature doesn't change much by season.

http://www.maintenanceworld.com/Articles/thibault/HowToRead/read_oil_can_tab2.gif

>Yes, heavier weight oil increase oil pressure but if it does not flow
>fast enough to replenish the oil between metal surfaces high oil
>pressure
>itself is useless.

It's not like there's an empty space (vacuum) in the bearings, just
waiting for oil. Once the oil is pumped up, it's oil displacing oil.
The reason oil pressure goes up is because it's harder to get the oil
_out_ from between the bearing surfaces to make room for more. There
are two issues with high weight oils: upon startup they don't pump up
as fast, which is why there is a "20W" spec (so it flows better when
cold), and they are of course more viscous, which doesn't mean they
don't protect from wear as well, but that they take more work to move,
which affects gas mileage. Higher weight protects from wear better, but
with more friction.