FWIW, I have always driven Vanagons, air-cooled and waterboxes, at between 55 and 62 mph on road trips.  Those slow
speeds (and low rpms) are, I think, easy on the engine, easy on gasoline, and give me time to enjoy the ride.

 I recently sold my 87 GL ("Long John") to a friend who decided to drive it at speeds between 70 and 80.  Long John is a
 very nice bus, has been very well-maintained, has 154,000 original miles, and has been extraordinarily dependable.
I suspect, however that its new owner ( and my close friend) will drive it into the ground within a couple of months.  I just
don't think these buses were designed for driving at high speeds.

I would like to install oil pressure and oil temp gauges, but I'm not sure where to begin.

I must say, I went through hell getting Billy Bones up and running, but I've driven mostly trouble free for about 10,000
miles now.   A trip from Oklahoma to Myrtle Beach, SC in May, and more than 5000 miles in New Mexico this summer,
all without incident.  Dependability is a nice thing.  These days when I go on a trip, what I experience is fun, not anxiety.

Warren Lail
88 Westy "Billy Bones"