ACEA A1/B1 Engine Oils

Engine oils that meet ACEA A1/B1 are meant for use in petrol and light duty diesel engines that are capable of using low friction, low viscosity oils with low high temperature / high shear characteristics (HT/HS).  This specification has become obsolete due to the fact that the new C5 specification covers the relevant low HT/HS oils while also ensuring compatibility with aftertreatment systems, that most modern cars have.  This specification should be used for older engines.