Sure Castrol GTX was used by me for many years, in my Aircooled engines, even the agents used it, it worked for me. But .......
Oil has changed a lot in the last 10/12 years or so, not due to the oil manufacturers but more because of the motor manufacturers. In some countries like US, Germany, it is law and oil needs to adhear to strict regulations & standards, this causing oil companies to fall in line. This brings about change, and what is actually in oil ? I have been looking at oil diffrently, I also said oil is oil, but have learnt that this is not true.
After having 2 not so stock engines recently give me problems due to rapidly worn out followers, promted me to start looking for answers. Btw I allways use the same formula in parts and assembly. A buddy who does engine building for a living has been having issues.
Our old engines were happy will fossil oils with lots of zink, phosphorous (ZDDP) and no detergents, todays oils have reduced levels of ZDDP this is done to protect catalitic converters and CO sensors as the zink kills them off, in doing so a lot of the lubrication properties needed for our old engines is lost. This is true as our engine need the zinc in the 2 areas of much sress, the cam & followers, and this also applies to many other vintage cars that run a similar setup.
In my search I found that ZDDP levels in Castrol GTX has dropped from around 1300ppm (ppm - parts per million) to around 800ppm, and from what I got to find out, ideal levels should be around the 1200ppm in our fossil motors.
So to make a long story short one should be using oils with a ZDDP level of around 1200ppm.
There are adittives out there that can be mixed in to boost the ZDDP levels, I have never been a fan of adatives of any kind so have decided that a chnge of oil in my motors especialy anything more than a stock engine.
By the looks of it, oils that are formulated purely for racing, as they don't have to worry about cat. converters is the way to go, as well as some other off the shelf stuff.
This is what I have found:
Mobil 1 15w50 full synthetic (1300 ppm ZDDP) this one is especialy formulated for flat tappet engines
Casrtol synthetic 20w50 (1200 ppm ZDDP)
Shell Rottela 15w40 (1200 ppm ZDDP)
Shell Rottela 5w40 full synthetic (1300 ppm ZDDP)
There are quite a few others but they are not all for sale here in SA, these mentioned above are rather pricey and the racing stuff even more.
The initial run in of a motor is where the engine is under the most stess and where the right oil can mean the diff. between it having a good life or a premature end.
