Here's the yawn, yawn bit.....it's a bit like watching paint dry.
The SAE tests oils and assigns an arbitrary weight or grade (viscosity) number to the oil depending on its flow rate at different temperatures. The SAE numbering systems goes: 0, 5, 10, 20, 30, 40, 50, 60. The grade refers to the oils ability to flow or lubricate an engine at a certain temperature. The lower numbers are thinner, less viscous oils suitable for low temperatures generally found in winter. The ‘W’ seen in an oil’s grade stands for ‘winter’ (eg 5W30)
0W oil will flow correctly up to -30 degrees C
5W oil will flow correctly up to -25 degrees C
10W oil will flow correctly up to -20 degrees C
15W oil will flow correctly up to -15 degrees C
20W oil will flow correctly up to -10 degrees C.
The flow rate of oils are also tested at a ‘hot’ temperature of 100 degrees C to simulate a running engine. The 2nd number in a Multi-grade oil refers to the ‘hot’ viscosity of the oil. So a 5W30 oil at 100�C will only thin as much as its higher rating. Think of it like this: a 5W30 oil is a 5-weight oil that will not thin more than a 30-weight oil when it gets hot. Multi-grade oil ratings look like this: SAE 5W-30. This is based on a 5W oil, tested at -25 degrees C, but at high temperature it will not thin out more than a '30' weight oil would.
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