Most of the time, when technical stuff like oil and oil viscosity are explained, the explanations become too technical for the average reader. This leaves some motorists to just ignore the numbers and details and use any oil that is readily available without knowing the adverse effects off using the wrong oil for your car or motorcycle. With this, we will try to explain oil viscosity ratings in layman's terms.
Simply, oil viscosity is the speed of flow as measured through a device known as a viscometer or, in layman's terms, the oil's resistance to flow around the engine. Think about cooking oil and honey. Cooking oil flows freely out of the bottle (low viscosity) while honey has more resistance when we spill it out of the bottle(high viscosity).
The thicker (higher viscosity) of an oil, the slower it will flow. You will see oil viscosity measurement in lube articles stated in kinematic (kv) and absolute (cSt) terms. These are translated into the easier to understand SAE viscosity numbers you see on an oil bottle.
What does 10w - 40, 5w-30, 20w-50 oil rating mean?
When you see a W on a viscosity rating it means that this oil has been tested at a Colder temperature less than the normal operating temperature of the engine. In the case of a 10w-40 rating, 10w is the cold rating.
The numbers without the W are all tested at 100° C which is considered an approximation of engine operating temperature. In other words, a SAE 30 motor oil is the same viscosity as a 10w-30 or 5W-30 at 100° C normal operating temperature. The difference is when the viscosity is tested at a much colder temperature. For example, a 5W-30 motor oil performs like a SAE 5 motor oil would perform at the cold temperature specified, but still holds the SAE 30 viscosity at 100° C engine operating temperature. This allows the engine to get quick oil flow when it is started at cold such until the lubricant either warms up sufficiently or is finally forced through the engine oil system. The advantages of a low W viscosity number are obvious. The quicker the oil flows cold, the less friction. Less friction would mean much less engine wear. Obviously, cold temperature or W ratings are tested differently than regular SAE viscosity ratings. Simply put, these tests are done with a different temperature system. There is a scale for the W, or winter viscosity grades and, depending on which grade is selected, testing is done at different temperatures.
In the Philippines, the W rating doesn't really matter that much as the Islands are usually tropical the whole year round. High performance engines do need low viscosity oil W to let it circulate more quickly at cold starts.
The next digits are basically operating temperature grade viscosity. During testing, using a viscometer, the heated oils viscosity is measured by labs and the amount of oil at 100° C is allowed to flow through an orifice and timed. Using standard data, they determine SAE viscosity based on different ranges. Thicker or heavy viscosity oils will take longer to flow through the orifice in the viscometer and end up in higher number ranges such as SAE 50 or SAE 60 for example. If oil flows through faster, being thinner/lighter, it will be rated at a low number range such as SAE 10 or SAE 20 for example. Occasionally, it is possible for an oil to barely fall into one exact viscosity range. For example, an oil is barely an SAE 30 having a time that puts it on the very low side.
Technically speaking these oils will be close to the same viscosity even though one is an SAE 30 and the other, an SAE 40. Meaning, there is not much that separates a ten point SAE rating in terms of viscosity. Point is, SAE 30 or 40 can be used on an engine that requires a SAE 30 in the owner's manual.
I can't tell you how many times I've been asked about which rating is better or a mechanic saying that they wouldn't use a 5W-30 motor oil because it is, "Too thin." Then they would use a 10W-30 or SAE 30 motor oil. At engine operating temperatures these oils are the same. The only time the 5W-30 oil is "thin" is at cold start up conditions where you need it to be "thin."
But I guess this lack of knowledge will continue as long as we don't educate ourselves on the products that we use. If you really don't care about this stuff, but you do care about your car, then it's best to read the owner's manual and follow it word for word, spec for spec.