Dear Tom and Ray:

My husband and I disagree about what grade of oil to use in his '99 Explorer. He refuses to take his car anywhere other than the Ford dealer because he gets mad when he sees others use 10W30, when the owner's manual calls for 5W30. It seems that everybody uses 10W30 at the non-Ford dealerships. I guess I should also mention that we live in Texas and don't get the cold winters that people get up North. But my husband claims that there HAS to be a reason other than temperature why Ford wants 5W30 in its engines. Why do they recommend 5W30? Give me a good answer, or I might have to trade in my husband before the lease is up! -- Linda

RAY: Well, we should first tell you what the multi-viscosity numbers mean. 10W30 means that the oil acts like a thinner, 10-weight oil when it's cold and like a thicker, 30-weight oil when it's hot.

TOM: So a 5W30 oil would be the same as a 10W30 once the engine is hot, but it would be thinner when the engine is cold (when it hasn't run for several hours). And that's why Ford, and most manufacturers, now recommend it.

RAY: A thinner oil flows and spreads more easily when you first start the car. And the faster it spreads, the more quickly it lubricates important moving parts. Presumably, that makes the engine last longer, because less friction damage is done during those first few seconds after starting.

TOM: How much difference does it actually make? We have no idea. Probably not a whole lot. It certainly makes less of a difference in Texas, where the outside temperature stays higher and the oil never gets quite as viscous.

RAY: So I'm sure it would not be a disaster if you used 10W30, but given a choice, I'd go with Ford's recommendation and use the 5W30.

TOM: And the reason a lot of shops use 10W30 is because it's the closest thing to a "one size fits all" oil.

RAY: Plus, mechanics love what it does to their hair.