Gaskets in an engine are sealing by the swelling of the material.
For static gaskets (like the o-ring on the oil filter cap), the gasket is clamped between the two surfaces. The engine oil coming in contact with the gasket makes it swell (mostly 10-15%). In this way, the gasket fits itself tightly between the surfaces to seal. (always watt a new gasket with some of the oil, so it can start swelling once assembled)
With dynamic gaskets (like at the kick-start-shaft coming out of the crankcase), it works in the same way. But here, the moving surfaces are not completely even, round or straight. The gasket needs an assured flexibility to follow the movement.
By time, the gasket material gets hard and so less flexible. The original swell might also be suppressed by subsequent compressing of the gasket (thermal expansion, pressure,…). The result is a small but steady oil leak.
Of course, to solve this problem it is clear that a new gasket has to be mounted. However stripping the engine might be time consuming and might not fit in our schedule at the moment.
You can try to save the situation by adding an additive to the engine oil: an “oil drip stop”. This product contains a so called softener. This chemical influences the hardness of polymers. It will make the gasket material flexible again. By increasing the flexibility, the gasket material will also be capable of taking up more of the oil and will swell again.
Be careful these additives are not miracle products! You can use it once. If it doesn’t solve the problem, you will have to strip the engine anyhow. Never use it twice as the product will make the polymers too “soft” and subject to extreme wear. Even if it solves the problem, consider this is only a temporarily measure.
Advice: Use an “oil drip stop” only once, for temporarily stopping an oil leak.