SAE 30 classic motor oil, 5l

This oil is recommended by VW for all aircooled engines. Modern oils contain dispersant additives that carry away contaminants which are then removed from the oil by a cartridge filter. Aircooled engines in older VW's dont use cartridge filters, instead they rely on a mash gauze filter which is unable to filter out this waste in the same way.
Morris SAE30 is a low dispersant oil thats been specifically developed to allow suspended debris to drop harmlessly into the base of your gauze filter.