In your feature “My Year’s Motoring”, in the February edition of Motor Sport, you mentioned that you run your 1953 VW on Castrol GTX motor oil. I, too, ran my 1968 VW on this oil, until recently I discovered that for some reason Castrol do not recommend this oil for use in VW engines. The only reason I can assume for this is that GTX does not have sufficient detergent qualities, as I did encounter sludging of the oil filler and breather while using it.
The recommended oil is Castrol CRI
Edinburgh. Douglas N. Thomson.