i currently use castrol gtx 20-50 in my stag. i am considering the use of synthetic motor oil. here in the usa the commonly available synthetic chioces are: mobil 1 15-50, castrol syntec 5-50 and pennsoil 5-50. i'm not keen on the 5-50 oils, so mobil 1 would be my first chioce. any thoughts on this?