OIL FILTRATION AND THE AIRCOOLED VW ENGINE
The oils we have available to use today are far superior to the oils that were around when the flat 4 air cooled engine were designed and in use by the masses. Today's oils have a myriad of additives and detergents that help to keep engines lubricated and clean internally. With the detergents the contaminants in the oil are broken down and help to prevent sludge build up.
That strainer at the drain plate in the VW does not count as a filter, its a strainer. It does not do anything to clean the oils in your engine. This is why you need an external oil filter with a modern full flow oil filter. The modern filters trap the contaminants that have been suspended in your oil and keeps them out of the internals of the engines.
WHICH FILTER TO USE
The smaller the micron rating of the filter the finer the particles that can be removed. Most "racing" filters do not filter as fine as a the non racing counterpart. This allows for more oil flow, but at the expense of finer filtration. Race engines are not typically run for thousands of miles before being torn down. Your classic VW in the garage on the other hand is realistically not being torn down and inspected after every drive or even every year. When looking for a filter don't cheap out, not all filters are equal. There are videos online of guys cutting open filters and showing the media inside. Here at DAMVolks we use and sell the Amsoil absolute efficiency oil filter, part number EaO15. Amsoil filters 99% down to 20 micron, keeping those particles out of your engine. See the image below for an idea on how big a micron is. A typical white blood cell is about 25 micron.