like anti-freeze, coolant, water, anything thats not oil. lol. water definately isnt good. after water has been in the oil for awhile it will turn to acid.
i would look at and fix (if need be) the cheapest things first. so you arent spending money if you dont have to. what year is your golf?
the haynes repair manual might have a trouble shooting section in it. if it does, look for over heating.
i did some googling on overheating VW's. most common things to check is water pump, thermostat, and head gaskets. i did see that the gaskets are not easy or cheap to replace.
found this:
QUOTE
First of all, lets try basic checks first before going into heavy tasks.It could be that the thermostat is not opening-faulty. Run the engine from cold for about 15-25 mins, have a hand feel of the upper and lower black water hoses (do this carefully water hose may be hot).If the bottom hose feel cooler than the upper hose, then thermo. is faulty. If that is ok, while engine is running, let somebody give the throttle a sudden burst and observe the hoses visually if they expand and contract suddenly. If this is the case, then you know your water pump is ok.Check your cooling fans, make sure second speed works (second speed is much faster). It could also be that your pressure cap on expansion tank is not able to raise the water boiling point. At this stage,if the above is all ok, i will suggest you visit a competent workshop for help.