quote:A5 Stable, stay-in-grade oil intended for use at extended drain intervals in high performance gasoline engines designed to be capable of using low friction, low viscosity oils with a HT/HS of 2.9 to 3.5 mPa.s. These oils may be unsuitable for use in some engines. Consult owner manual or handbook if in doubt.
Mobil 1 0w30, 5w30, and 10w30 are all A5 rated oils.
Dr. T I believe "may be unsuitable for some engines" when refering to the ACEA A1/B1 and A5/B5 is warning that the oil may be too thin to use in some motors. An A2/B2 motor oil would not come close to meeting the requirements of a high performance motor oil such as A3/B3 or B4 and as such may not be suitable in a high performance motor.