Castrol oils, particularly GTX, were always noted as being very high detergent oils. A common warning was not to switch to it after using low detergent oils as it would clean up the system too well and either load the filter system, or drag sump contaminants through all the bearings.
I used it exclusively on my VW's and noted just how pristine the case and sump areas were compared to other engines not using it. Some preferred not to use it, particularly if a filter wasn't used, as the theory was that contaminants were carried within the oil (albeit captured and coated) if not captured by a filter and not settled out at rest. Can't comment on that, but it always gave me great service and I only stopped using it in later years when GTXII came out with a lower viscosity 10W50, and my car (Toyota Corolla) started using oil. I now use Valvolene XLD 20W50 in the car.
Bill....................;-)
