Straight 50-weight is
really thick. It will probably have a hard time getting everywhere it needs to when the engine is cold. I wouldn't use it on anything that didn't very specifically call for straight 50.
20-50 is an oil that is as thick as a 20-weight oil is when both are at 32F, but as thick as a 50-weight oil is when both are at 212F. It thins out less than straight-weight oils when warming up, but it still does thin out. It is a much better choice for our motor than straight 50-weight.
The original spec was for 30-weight, but back in the late 60s when our cars were first being built the multi-grade oils really kinda sucked. 20-50 seems to be a good default choice, unless your engine builder set up the internal engine clearances and such for some other weight of oil.
BTW, the 914 in near-stock form seems to work best with equal size tires front and rear. As a bonus, you can rotate the tires front to rear to even out any wear imbalance. I like 195-width all around, though the 205s on my car now are slowly converting me. (They're Falken Azenis, though--the grippiest hard-compound tire you can get.
Thanks, Grant!!)
--DD