What would cause oil to leak in the distributor and how can it be fixed Nissan pintara 1991?

With the engine running, remove the oil fill cap and hold a piece of paper over the hole. If the paper is pushed out of the way it's an indication of "blow-by". That's a condition where exhaust gas is leaking past the piston rings, into the crankcase. If pressure builds in the crankcase it will blow oil past the distributor seal and can cause front and rear main bearings to leak as well. The condition is often caused by failure to properly maintain the engine, either infrequent oil changes or air filter changes or both.