Hi Scotty,
I'll have a go but it takes a while to write these things out....
Imagine that you have five buckets of oil, all different weights, a 10, 20, 30, 40, 50. The 10 will be quite light, and each will get heavier until the 50 is very heavy.
Now put these buckets of oil over a heater so they heat up to "half hot". They will each be slightly lighter than before. The operating weight of them as they thin out is now 5, 10, 15, 20, 25.
Now keep heating them up to as hot as an engine at full running temperature. They get even more thinner so they are now 3, 6, 10, 14, 20. It might help to write these numbers under each other as they get hotter so you can see the progression.
What we have is a lower viscosity rating for each oil as the temperature rises.
But this is not good for engine oil as we want it to stay the same through the operating range. If we need 20 grade oil so we can actually pump it around the motor without taking too much of the motor's power away, that is what we need. And we need it when the motor is cold or hot.
Trouble is, we put in cold 20 grade oil and it won't keep the thing properly lubricated when it is hot and is thinned down to 6 grade. But if we put in 50 grade oil which works OK when it is hot it is too stiff for us to pump around the motor when cold.
So the oil gurus came up with methods to keep the oil at certain vicosity levels. They do this by combing different length molecules with different characteristics of how they join up with each other. Those stable oils are what we use these days.
But when they did this they wanted a method of describing how viscous the oil was when it was cold and when it was hot. So what we have now is a cold/hot method of describing oil. Forget the W, but 20W50 means at cold it is 20 grade oil - and when it is hot it is still 20 grade viscosity, just as a 50 grade oil would be when that is hot.
They could have just cal
led it 20 grade, but some things like oils that thin radically when hot, and sometimes we need to hold a fairly level viscosity through a wide temp range.
As the Castrol ads say (at least here in Oz), "Oils aint oils".
This might still be a bit confusing. Sorry about that. In people terms we might say something like, "You're pretty normal most of the time but behave just like your father when you get under pressure." This might mean we lose our temper or we cope wonderfully.
When the heat's on, we start to behave differently. So does oil.
I'd better stop before this gets too confusing.
Anyway, what time are you over there? Here in Oz it's 8:50 Thurs night. We got Star Wars II showing 20 hours ago.
Kim
i'll let you know how it works out. thanks, bill
