If you have water (coolant) in the oil, the first place you will see it is on the underside of the oil filler cap, you will see this ugly light brown goo when you take the cap off and look at it, if it just looks oily you should be ok on that score. Oil pressure switches like to leak when they get old and can pump out a significant ammount. Head gaskets often weep oil on the exhaust side where they meet with the block. That's usually a grin and bear it, it isn't worth it to pull the head and replace the gasket. Coolant leak could be heater hoses or valve, but may only leak when hot or under pressure and can be tough to find. Mike W.