I have a 1984 Volkswagen Westfalia Camper with a 1.9 watercooled flat four that is in the process of being rebuilt. I was using synthetic before the engine rebuild and the engine did not leak more than a few drops here and there. It ended up with severe blow-by due to rings/cylinder wear. I have only had the van for a couple of years, so the internal engine condition was an unknown. Some of the Volkswagen group that I have talked to say that synthetic oil should not be used with an older engine design as it will leak and have blow-by. They recommend Castrol 20-50. Most are air-cooled VW owners and my guess is that their recommendation is based on the fact that the air cooled engines may be set-up with looser piston to cylinder tolerances to allow for higher operating temperatures. I have used synthetic in all my other vehicles and don't want to go back to the dino oils. I reason that 15-50 Mobil 1 is going to be a superior to a 20-50 dino oil. I understand that I should use a dino oil for the first 2,000 km or so to allow for proper break-in. Do you think the 20-50 is too heavy for break-in? The weather is between 20-30C and the little 1.9 works pretty hard in a 5200lb GVW vehicle.