1994 Volkswagen Golf I have problems with my car (oil light

Were they white crystals on the cap? If so you may have blown your head gasket, especially if the mechanic found anti freeze in the oil. Gas in the oil is highly unlikely as the two only have the chance to mix in the combustion chamber and if they do mix there it is usually the oil getting burned with the gas and resulting in blueish gray smoke and is caused by bad piston rings or a severely blown head gasket. The only way I can think of gas getting into your oil is through the piston rings and that would cause a severe loss of power in one or more cylinders.

It is finally prestone that was being mixed in the oil.
If I want to fix this, is this something that can be done easily, or someone told me you are better off changing the whole engine when this happens to avoid trying to fix a problem that may iteself cause other problems.
If it was a head gasket, is it something that once fixed is fixed, or could lead to other problems because of the connection to other motor parts.
Thanks in advance

About 90% of the time it is a head gasket that causes this, the other 10% is strange things like leaking from manifolds or turbos.

As for a head gasket fix, this one is easy, the only thing that usually happens to be a problem is the warped head, your block is cast iron and does not usually get damaged by a blown head gasket. Non the less every thing has to be checked with a micrometer and the head will have to be machined flat again.

If this turns out to be a head gasket and you like the car it is most certainly worth doing as there are relatively few problems after it is done and with so few miles on the engine it is worth keeping.

As for changing the engine out, for the price you can buy a new car, don't bother unless you want to throw in a racing engine.

Speaking form lots of experience on this you can get away with doing your own head gasket for a good days work and about $300, and that early Volks Wagen engine is very easy.

To confirm your mechanics prognosis, check the oil yourself and see if it looks like chocolate milk, if it does that is antifreeze.