[beetle1303]

the reason that synthetic oil feels slicker is that its more refined and there are many additives into it in order to provide the viscocity properties.
Something that i havent confirmed yet but its been in my mind on the matter, is that a magnesium case (T1) is not of the high tolerance like an aluminum one like the T4. Thus, its molecular stucture is not so thick as in aluminum (plain words its like having more pores not only on the surface, but on the structure as well). This along with the more refined structure (thinner) of the synthetic oils can cause leaks. Im not the expert on the matter, but i have done a bit of reading.
Currently im using Pennzoil 10-40 on my T1 and i can say that i dont have any oil burned up (no blue smoke or anything) and the oil my engine is loosing in my opinion is due to poor machining/worn out case/exploded rubbers on the stock oil cooler/ and blown head gaskets every once in a while (cos im not running breathers on the engine...)