Beware, excessive viscosity and resultant excess oil pressure can cause the oil pressure relief to activate and bypass oil around the oil cooler, resulting in elevated operating temperature. Remember, an "aircooled" VW engine is in truth air and oil cooled, hence the need for the oil cooler. The oil pressure relief is designed to only operate when the engine is cold and viscosity/pressure is high. At temperature in normal operation you should be bypassing little to no oil around the oil cooler. I know lots of people run 10W40 weight oils in ACVW engines, but I think this is more of a legacy of the days when the viscosity index improvers would shear down fairly quickly in the early days of multi-viscosity oils. VW recommended either straight 30W or 10W30 under most operating conditions in these engines originally, and if I were going to run a high quality modern synthetic motor oil (which will not shear down much or shear quickly) I would stick with the original 10W30 or perhaps a 5W30. I've definitely seen cases of elevated oil temperate cured by dropping down to an XW30 grade oil. You might get by with a 10W40, some do. Most will see elevated oil temps when running a 20W50.
When it comes to viscosity in these engines, more is not always better.
