SAE 5W, 10W, 15W and 20W ratings are oil thickness (viscosity) measurements at –25°C, -15°C and –10°C respectively. This is all to do with cold starts in winter conditions. 5W and 10W are really intended for severe North European (or American) winters.
DES - The Basic Facts:
SAE 20, 30, 40 and 50 (60 is an obsolete spec originally for large air-cooled aero engines) all measured at 100°C, a typical sump temperature in a hardworking engine. SAE 50 gives the best protection in hot climates where oil temperatures may exceed 100°C.
Once an engine has reached its working temperature (if over 70°C) the SAE "W" rating has no effect on the engine.
Wide-range multi grades (5W/50 etc.) were originally intended for severe climates where cold starts down to -20°C could be expected, followed by prolonged high-speed motorway use. The normal lubricants for moderate climates are 10W/40 (Northern Europe) and 20W or 15W/50 (Southern Europe). Apart from cost, wide range multi grades have a very high polymer content and can suffer from "shear down" effects (loss of oil viscosity during use). It is interesting to note that "MOBIL I RACE", specially aimed at the motorcycle market, is a 15W/50 so as to handle small high-revving motors where the MOBIL I is 5W/50 [now 0W/40, 1998].
A "synthetic" oil does not automatically guarantee extra engine protection. Those based on synthetic hydrocarbons (PAOs) last longer, but give no better wear protection than a mineral-based oil. Those containing the more expensive synthetic esters also contribute to anti-wear performance; A 10W/50 "pro-4 race", based on a 100% ester/PAO mixture, is near the end of its development programme, but it will be expensive. It is mainly intended for "super-bikes".
SAE RATINGS - The Unadorned Facts:
The American Society of Automotive Engineers defines two categories of engine oil viscosity, one which deals with high temperatures typically reached in a modern engine, and one with winter start conditions. SAE numbers 20 to 60 cover five viscosity ranges based on measurements at 100°C. Various types of laboratory kit are used to measure viscosity, but the answer comes out in standard units known as "Centistokes"(cSt).
SAE 20 covers 5.6 to below 9.3 cSt at 100°C
SAE 30 covers 9.3 to below 12.5 cSt
SAE 40 covers 12.5 to below 16.3 cSt
SAE 50 covers 16.3 to below 21.9 cSt
SAE 60 covers 21.9 to below 26.1 cSt
"W" ratings are only relevant to sub-zero cold start conditions.
"W" stands for Winter. (Not "weight" as saloon bar experts will tell you).
A 0W has to be below 3250 cSt viscosity @ -30°C
A 5W has to be below 3500 cSt viscosity @ -25°C
A 10W has to be below 3500 cSt viscosity @ -20°C
A 15W has to be below 3500 cSt viscosity @ -15°C
A 20W has to be below 4500 cSt viscosity @ -10°C