Why is oil pumping into air cleaner on your 88 fxrp?

It's probably caused by "blow-by". Blow-by is caused when the pressurized exhaust leaks past the pistons, into the crankcase. The moving gasses pick up oil vapor. The exhaust gas dumps into the air cleaner and if the engine has blow-by the exhaust gas the oil will deposit in the air cleaner. So to make it short, blow-by means the engine is worn, it's not a good thing. But not necessarily. The FXR, like many other H-D designs, utilizes a dry-sump oiling system. In other Words, oil is not stored in the bottom of the engine, as is the case with most cars and SUVs. After it flows through the engine and does it's work there, it drains into the bottom of the engine, and is pumped from there to a tank, and back to the engine. When the engine is off, the oil is held in the tank and prevented from draining back into the engine by a little check ball. If that ball gets dirty (often caused by infrequent operation of the vehicle and neglected oil changes), it does not seal properly. Oil can then drain past it into the bottom of the engine. It does no immediate harm there, but the next time the engine is started and the pistons start moving, there is a change in pressure within the engine that forces this oil to kind of "burp" out of the vent line which leads to... you guessed it, the air cleaner. Change the oil, clean and flush the tank and lines, filter screens, and said check valve, and you'll be fine.