Multi-viscosity

Multi viscosity oils are one of the great

improvements in oils, but they should be

chosen wisely. Always use a multi grade

with the narrowest span of viscosity that

is appropriate for the temperatures you

are going to encounter. In the winter base

your decision on the lowest temperature

you will encounter, in the summer, the

highest temperature you expect. The

polymers can shear and burn forming

deposits that can cause ring sticking

and other problems. 10W-40 and 5W-30

require a lot of polymers (synthetics excluded)

to achieve that range. This has caused problems

in diesel engines, but fewer polymers are better

for all engines. The wide viscosity range oils, in

general, are more prone to viscosity and thermal

breakdown due to the high polymer content.

It is the oil that lubricates, not the additives.

Oils that can do their job with the fewest

additives are the best.

Very few manufactures recommend 10W-40

any more, and some threaten to void warranties

if it is used. It was not included in this article for

that reason. 20W-50 is the same 30 point spread,

but because it starts with a heavier base it requires

less viscosity index improvers (polymers) to do the

job. AMSOIL can formulate their 10W-30 and 15W-40

with no viscosity index improvers but uses some in

the 10W-40 and 5W-30. Mobil 1 uses no viscosity

improver in their 5W-30, and I assume the new

10W-30. Follow your manufacturer's recommendations

as to which weights are appropriate for your vehicle.