I have no expertise concerning the use of 60 wt oil, or for that matter any
oil.  My behavior for any vehicle has been to follow the manufacturer's
recommendation.  The owner's manual for my 1991 Volkswagen Vanagon GL
Campmobile recommends 20W-50 oil over a wide range of temperatures.  I have
finally switched to 15W-50 Mobil 1, and pressure at high temperatures seems
a little better (doesn't drop below 10 psi according to my gauge) on long
high ambient temperature runs.  It also seems to drop to around 40 psi more
quickly from around 70-80 psi at very cold startup compared to when I was
running the 20W-50.

Is there a reason, despite the owner's manual recommendation, that I should
be leery of multi-grade oil with the higher number being 50?  My vehicle
only has 189K miles on the waterboxer, it doesn't use oil, and once I got
the cooling system and the oil cooling system sorted out, pressure seems
fine.  When I first got the vehicle, was even more ignorant than now, and
hadn't yet done the cooling system and oil cooler work, pressure was low on
long runs at high ambient temperatures.