Which motor oil to use in your classic Volkswagen
This question comes up all the time. I use 15W/50 Basic Mineral Motor Oil. The John Muir book recommends the best monograde, heavy duty (HD) military standard (MS) oil you can buy. I don't know anywhere that still sells monograde oil. Some books also recommend a basic 10W/40 oil, which I also can't find on the shelves, but if you can, go for that.
It's very important that you go for a 'basic' oil for your engine. Modern oils such as the 'Magnetex' range are built for modern, fuel injected engines. An aircooled engine is using the oil for cooling as well as lubrication, so it needs an oil suited to the age when the car was built. These days that means looking past all the fancy labels and flashy graphics, look for something plain then read the label. Find the one that says it's fine for older cars. Mine says "Formulated to protect older engine designs. Suitable for most pre 1980 vehicles." That's the sort of thing you need to go for.
If you live in a very cold (Siberia, parts of Canada, etc) or very hot (Columbia, Iran, etc) area these oils might not be right for your conditions. Ask in a local garage about what oil will be suitable. Don't worry if it's not a VW garage (though if you can find one, great) just try and find a mechanic that looks like he's been around a while and ask, he's bound to have worked on an old Volkswagen at some point.
Amount of oil
A Type 1 engined Beetle, Van or Karmann Ghia takes 2.5 litres (metric) which is 2.6 US quarts, Type 4 engines take 2.8 litres / 3 US quarts of oil.