I have recently discovered that there is oil on the radiator cap. I am driving a Toyota corolla with 4A engine which is automatic. What could be the cause for that?
Another possibility would be a cracked head.Either way,leaking head gasket or cracked head, not a good thing. Also if any oil cooler lines go into the rad that would be another possibility, so maybe just the rad would need fixed or replaced. If head comes off to change gasket, check the head for cracks.
If there is oil on the radiator cap, you have to assume something near the cap is spraying oil such that it can get up onto the cap itself. My suggestion is to look for hoses or fittings nearby that could have a minor leak under pressure that makes a lot of mess in the area.
For example, I recently replaced my aging power steering line: it had a small hole that was spitting what looked very much like engine oil into a localized area. Replacing that $30 USD part solved the messy problem.