Such lubricant compositions are also referred to as SAE 0W-x compositions. SAE stands for Society of Automotive Engineers in the USA. The “0” number in such a designation is associated with a maximum viscosity requirement at −35° C. for that composition as measured typically by a cold cranking simulator (VdCCS) under high shear. The second number “x” is associated with a kinematic viscosity requirement at 100° C.
The minimum high temperature viscosity requirement at 100° C. is intended to prevent the oil from thinning out too much during engine operation which can lead to excessive wear and increased oil consumption. The maximum low temperature viscosity requirement, VdCCS, is intended to facilitate engine starting or cranking in cold weather. To ensure pumpability the cold oil should readily flow or slump into the well for the oil pump, otherwise the engine can be damaged due to insufficient lubrication. The mini rotary viscosity (MRV) requirement is intended to ensure a minimum pumpability performance.
U.S. Pat. No. 5,693,598 describes a lubricant formulation according to 0W-20 based on so-called poly-alpha olefins. Poly-alpha olefins are prepared by oligermization of alpha olefins (PAO) as for example described in U.S. Pat. No. 3,965,018. The disadvantage of such PAO base stock is its high cost of manufacture, as for example mentioned in the introductory part of U.S. Pat. No. 6,060,437. Nevertheless PAO are widely used to formulate such lubricant compositions because no commercially available alternative exists having the cleanliness and low temperature properties as PAO. Another aspect of using PAO base stock is that additional base stock like for example ester based or aromatic based base stock will also be present in the lubricant formulation in order to confer additional desired characteristics as for example additive solvency and seal compatibility.