Dear Tom and Ray:

What is the primary reason why car manufacturers have switched to lighter-weight oils? I've heard several explanations. But once and for all, what is the real reason for this change? I would feel truly blessed to receive your thoughts and opinions relating to this question! I know you guys have the answers. Thanks. -- Dan

RAY: They've switched over to lighter-weight oils because the guys loading in the cases at Kmart were continually complaining of backaches, Dan.

TOM: Actually, the main impetus for using lighter (lower-viscosity) oil is to improve gas mileage.

RAY: The thicker, or more viscous, the oil, the harder it is for the moving parts of the engine to "push" their way through it. The more resistance those parts get, the more fuel that is required to make them move. So, by lightening up the oil, you make it easier for the parts to move, and you reduce your fuel consumption.

TOM: It used to be that thicker oil was used because it provided more protection to those moving parts, especially at higher temperatures, when all oil thins out. But oil technology has improved so much that today's 5W30 oils provide even more protection than yesteryear's thickest glop.

RAY: It's also true that the tolerances (the spaces between parts) have gotten smaller over the years as quality has improved. And those big, old, fat, porker oil molecules can't get in there as well as the thinner ones.

TOM: In the old days, you used to be able to fit, say, a Maine coon cat in the spaces between the bearings on an old Chevy. But today the spaces are measured in fractions of a millimeter. And today's thinner oils are exactly what's needed to get into those spaces.