Even with Diesel's RPM's are important. They have a lot more torc but they have to be in the right RPM range to be effective. If you lug a diesel meaning the RPM's are too low for the grade you can watch the temperature rise.
The only time I have heated the motor after I had experience was climbing a couple of grades in Death Valley. It was 10:00 in the morning and 130 degrees. Everytime I would try the climb I would only get two miles and the temperature would rise to 200 degrees. I would just pull off to the side, put the rig in neutral and rev the motor. The temperature went down immediately.
We pulled the entire Teton Pass, out of Jackson Hole, 12 miles at 12% grade and never did heat. I was pulling at 2200 RPM's which is right for our 300 cummins.
200 degrees is hardly enough to be worried about pulling over for a cool down. Oil at min needs 180 degrees to burn off condensation. Most pre-mixed coolants at 50/50 mix won't even begin to boil until 275+ degrees.
I see you have an air-cooled VW bus...still own it? Which motor T-1 or T-4. Know how hot they run? The heads along can get 400+ degrees. The Type-4 engine is miles ahead of the T-1 when it comes to overall reliability and longevity. I own a 1972 Westy hardtop with a 2056cc Type-4 engine.