I have done extensive testing of oil temperatures vs. oil level in my 90
Vanagon syncro and my 87 vanagon syncro. I use a VDO gague panel I had
installed, which shows the oil temperature. 

I can tell you that the oil temperature when the engine is full of oil is so
high I almost stopped driving and drained it off into the bushes with my wife
waiting in the van. The best temperature was always obtained when the oil
level was at the minimum setting. 

I would never, ever, drive my van with the oil at the maximum setting again.
This has been confirmed on both my vans. I have all the temperature readings
here on my hard disk somewhwere but I am afraid to quote them or fear of
getting them wrong.