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Fortunately, we have the time of single-grade oils long behind us. A single-grade motor oil is like a much too short bedspread. You must decide whether the bottom or top is covered. For the engine this means; either it has reasonable cold starting characteristics or a good wear protection at high temperature. Both simultaneously is not possible to accomplish with a single-grade motor oil.

A 10W single-grade oil has only a kinetic viscosity of approximately 5mm2/s at 100 degrees Celsius. The HTHS value of a “W” single-grade oil is not identified but for a 10W oil it would be at 2.0 mPas as max, more likely somewhat lower.

Because of that you had to decide which oil to use and change motor oil quite often accordingly to the outside temperature. At temperatures below 0 and shorter journeys e.g. a 10W, for the transition period in spring e.g. an SAE20 and for the real warm months an SAE30 or 40.

If an SAE30 would’ve been used at temperatures of e.g. -15 degrees Celsius, the engine wouldn’t even have started, accordingly the engine would have been broken by using a 10W oil in the summer at high temperatures. The first multi-grade oils were still a bad joke compared to todays high-performance multi grade motor oils but a significant step forward.

A multi-grade oil is a multi-grade oil only if it meets the requirements of a “W” class oil in cold state and of a class without the “W” at 100 and 150 degrees Celsius. If we take a look at a 10W-40 oil for example, this oil behaves as a 10W oil at -25 degrees Celsius and like an SAE40 at 100 and 150 degrees Celsius. Furthermore it has to have a kinetic viscosity of 12,5 – 16,3 mm2/s and an HTHS of minimum 3,5 at 150 degrees Celsius if is classified as an ACEA A2/B2 or A3/B3. To accomplish this a VI of about 150 is necessary which is unreachable for a single-grade mineral based oil (they have about 90-100), that’s why multi-grade oils were not possible in the past.

Wintertime the average oil temperature during engine operation is lower, how much lower depends on the outside temperature and how much load is applied to the engine. It’s nearly no difference if it has +5 degrees in the winter or +15 degrees in the summer, if it differs from -15 degrees to +30 degrees makes already a difference for the oil.

Did you have a 10W in your engine and the temperatures rose significantly over zero degrees Celsius you had to drive very carefully and change your motor oil as soon as possible – something we fortunately don’t have to think about anymore.