There is a check valve for the oil feed that is located in front of the crankcase, on the right side, under the water pump. It'll look like a drain plug that sticks out towards the front to the machine. Behind that plug is a spring and a check ball that could have a problem. The check valve is used to keep oil from draining out of the external tank, into the crankcase.
It's also worth a shot to do a compression test to see if you have excessive blow-by, specs are 85-90 PSI. Check the oil return line and vent hoses to the external tank as well.