Most of those 19.5s were the over head valve single cylinder. If that is what you have,You either have a blown head gasket, or worn rings. Either way, you are getting too much combustion pressure in the crankcase and it is forcing oil through the crankcase breather, which connects to the intake, where it runs any oil that gets through back through the engine aand burns it.
It would be worth the cost of the gaskets to check the head gasket. They have a tendancy to blow between the pushrods.