The wonderful world of oil.
The most important factor in oil is the viscosity. This is a number which indicates the relative thickness of the oil. The higher the number the thicker the oil.
Unfortunately thin as well as thick is not good. Thick oil is hard to pump through the engine and does not make a nice film. So thinner oil is better. But... when the oil is to thin the oil-film (which is what it's all about) might rupture or break, leaving the bare metal unprotected.
A difficult problem, how to choose?
The sae number give's you two (actually three) pieces of information:
The first number is the base viscosity. Usually followed by the letter W which means this viscosity is at 0 Fahrenheit or 17 Celsius. (so, when the engine is cold)
The second number (behind the W) tells you what the theoretical viscosity would be (cold) for an oil that is as thin (or thick) as this oil at 100 Celsius. So, the higher this number the less thinner the oil will get when it heats up.
Conclusion:
If you have to deviate from the manufacture’s proscribed SAE, the first number is the most important one. If you have to use 10W40 and you would use 20W40 you can expect problems with the clutch and excessive engine wear when cold.
On the other hand, if you choose an oil like 10W30 the oil might get to thin and break up when the engine is hot.
In the case of choosing 10W40 instead of 20W40 you have an oil which is thinner when cold, but not to thin when hot. So.. it’s even better then the original.
One extra interesting piece of information is the fact that manufacturers service intervals are determent for mineral oil (or dead dinosaurs juice). Synthetic oil will last 2 or 3 times this period.
Maarten
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