I've been running 15w40 in my 49 216 since I got it running again (approx. 15k miles ago) and it avg's 12/13 psi cold at speed and around 7/8 psi warmed up at speed. Idle is around psi 5 or so.
That's warm weather pressures.
With colder weather it'll register 15 psi cold but will still settle don to around 7/8 psi when warmed up, just takes longer to warm up.
re. mixing oil. I mix in 20w50 (when I find it on sale)over the summer and get a psi or two higher but stay w/15w40 over the winter. My 216 leaks out just about every sealed orifice when running so I mix oils on the fly (always Rotella T) and theres no telling what the actual "weight" is
(it's some combination of 15w40/20w50) The 216 likes whatever the concoction is and so far hasn't let me down.
Ditto on Jerry and Denny