Dear Jayson;
Yes many years (50) in the business.
They "work" but don't lubricate as well as 'standard' SAE 30 does. My guess would be that the 'early' cast iron engine was not designed for the multi grade (lighter/thinner) oils which began as a US Army idea in WWII.
Now; in a 'freezing' climate, not so. SAE 30 is pretty thick (at start up) for that usage.
You need to remember; in that era there weren't people running around doing "study(s)". We just did what worked best for us and others in the industry..
From the early 50s through the early 60s all commercial vehicles and most passenger cars used 30 weight oil. Most 'racers' used Valvoline 30 on & off the track.
By 1963 the engines changed to the 'full flow' oiling systems. Today many use multi-grade oil in them, for all climates with great success.
Try this test. At room tempature put a teaspoon of 30 weight & one of multi grade on the same incline. The multi-grade will run off the table while the SAE 30 stays a while.
Try Dello 400 SAE 30 in one of yours and see.
