I have been reading up on my 38 Master project. I understand that the 216 has an oil slinger, pre-dating the rear main seal. How effective is that design at keeping the oil in?

The slinger works fine as long as the engine is running. If the volume of oil is too great some will leak past the slinger. It is a good thing as it alerts the owner that their is a problem with tolerance of main bearing or plugged drain hole.

I have had fewer problems with my cars with slingers than the cars with seals. Important to have everything "correct" as Chipper mentioned.

Interesting post! Some time ago I drove my truck at "highway" speeds of 55 to 60 mph. After about 15 minutes I noticed a scorched oil smell and light cloud of smoke. I pulled over and found the previously clean engine bay had oil scattered everywhere. The temperature and oil pressure guages registered normal. I added about 8 oz of oil. Before starting the trip all fluids were checked and topped off as needed. I let the engine cool for about 15 minutes and got back on the road. I held the speed then and since to 50 mph or less. Since then I have driven about 500 miles and not had another incident. Could this be a plugged drain or main bearing problem?

NO - A rear main leak will not get the engine compartment full of oil.
You may have a rocker arm cover gasket leak, side cover gasket or FUEL PUMP. I have had three different fuel pumps leak oil at higher speeds. The lever pivot pin leaks where it goes through the casting.

I have had a fuel pump leak at the lever pin too. It was hard to see as at idle it didn't leak. At higher speeds on the road the oil was blown along the side and bottom of the engine. One day I ran it at a higher RPM in the garage and after a minute or so I could see it seeping out from the bottom area of the pump.

Just to add to this discussion, the four speed transmission I nmy 1951 truck has an oil slinger at the front, with no seal. It is very sensitive to having the correct level of oil.

The 1938 and up 3 speed also has a slinger nut and no seal. They don't leak unless there is something radically wrong.

Thanks guys.