Last week I got my first oil change at wal-Mart. Requested Mobil-1 5-20 but they were out. The only syntethic oil they had in 5-20 was Castrol. Had them put it in. By the end of the first tank of gas there was a noticable smoothing out of the engine. The car (Ford Focus 2005, 4600 miles) runs smoother, starts easier. Found this site after the oil change. Didn't know that Castrol was not a true syntethic. Looked at Amsoil site and there comparison of oils and the Castrol looked pretty close in most of the ratings to Mobil 1. Think I will stay with Castrol for a while. Also was stuck with fram filter $11.16!!! ouch!!!