The age old question, which oil. If you have an air cooled engine, the air is cooling the engine, but that is not the end of it. To get the heat out to that air needs a conductor of heat, namely oil.
A thin oil will not transfer the heat and as the heat rises, it becomes thinner, conducting the heat less effectively. Use a top quality mineral oil, Volkwagen recommend 20W50. Ensure that it is topped up to the correct level, checked regularly and that the filter is changed every 3,000 miles.
Add up how much oil you add in between oil changes, an air cooled engine is supposed to burn oil. If you are exceeding 1 litre / 2 pints of oil every 3,000 miles (see the VW handbook which confirms this), then look for visible leaks and smokey exhaust gases. It may be time for a rebuild.