Today’s oils are products of modern technology and
chemistry. The thinner oils perform much
better than the blends of yesteryear under high temperature conditions;
viscosity breakdown is no longer a serious problem; and the overall film strength
is superior. However, the thinner oils
are really designed to compliment modern vehicles built since the mid-80s. Car manufacturers tolerances are a lot
tighter. The engines are practically
blueprinted right out of the factory compared to the sloppy tolerances of 20-30
years ago, and internal clearances have been reduced to take advantage of the
lighter oils. Also the advent of fuel
injection has allowed consumers to drive the cars as soon as they fire up. Thinner oils help with cold running
wear. Also, with fuel injection there’s
less oil dilution by overly rich mixtures leaking down into the crankcase past
the rings. I would not use 5W- or 10W-30
oils in old tech big blocks, although a premium 20W-50 is more than adequate.
Conversely, thick oil conceivably may cause problems in
winter months on late model cars with their tighter internal clearances. In the absence of specific manufacturer
guidelines to the contrary, 76 Performance Products recommends 5W-30 for
northern-tier states; 10W-30 is preferred for south Texas and similar climate
areas. After 100,00 miles step up in
viscosity to 10W-40.
Any fuel benefits from today’s slicker multiviscosity
oils are generally negligible, on the order of 0.1 mpg or so. That’s no big deal to guys like us, but it’s
significant to OEM manufactures forced to meet government fuel economy
standards. For the purposes of CAFÉ
(Corporate Average Fuel Economy) calculations, a model’s fuel mileage is
rounded off to the nearest whole number.
For example, a car that averages 18.45 mpg is considered an 18-mpg car
for the government’s purposes. If
thinner oil improves its mileage by 0.1 mpg to 18.55, the mileage rounds up to
19 mpg, which is significant over hundreds of thousands, or even millions, of
units when calculating the entire average mileage performance of a given
manufacturer’s new car fleet.