Many, many years ago oil was so clean it looked like the lightest
color maple syrup when it poured out of the can. You could make a
rough guesstimate at when an oil change was needed per the darkness of
the oil. Cars had a lot more blow-by and the oild got darker the
longer it was driven. You cannot do that anymore. You need to figure
out how your driving fits into the manufacturer recommended
maintenance schedule in the owner's manual; most drivers actually fall
into the severe duty maintenance schedule. Change your oil (at a
minimum) per that recommended schedule. If your car has an oil change
sensor you can use it to track the need for changing. They are quite
accurate at tracking the time, mileage and condition of oil.
Now for an oil history lesson.
I remember my Grandfather getting his oil changed on his big old Buick
Roadmaster every 1000 miles along with greasing all the points on the
chassis. This was using the original motor oils that were around at
that time. You put 20 or 30 weight in in the summer and 10 or 20
weight in the winter. You also threw a blanket over the hood to retain
the engines heat in the winter but I digress....
As time went by manufacturers started putting a LOT more additives
into oil. Shortly after WWII, the American Petroleum Institute (API)
developed a system that established three basic types of engine oils:
regular, premium and heavy-duty. Because three oil classifications
could never hope to cover all of the different applications ranging
from conventional passenger cars to heavy-duty trucks the API
realized that other variables had to be considered, such as the type
of engine and its usage.
In 1952, the API launched the service classifications system.
The API system revolves around two general classification: S for
Service (passenger cars and light trucks) and C for commercial
applications (smelly yucky diesels). The breakdown of "S" varieties is
as follows:
SA: Plain mineral oil that doesn't contain any additives. This oil was
primarily used in the 1920s - nobody uses it today.
SB: Lubes that contain anti-wear and oxidation inhibitors as well as
corrosion inhibitors. This oil was primarily in use prior to 1964 and
was created for vehicles that saw moderate conditions.
SC: This classification was originally recommended for use in 1964-67
vehicles. It contains additives that control rust, wear, corrosion and
engine deposits. It too is now obsolete.
SD: SD lubes were recommended for use in 1968-70 vehicles as well as
certain post-1970 passenger cars. This oil contains the same additive
packages as the SC class and can be used in place of it. (When I
started changing oil it was rated SD on the cans).
SE: This category was recommended for certain 1971 vehicles as well as
most 1972 vehicles. This classification offers more protection than
the SD group of lubricants and is suitable for severe-duty
applications.
SF: Recommended with 1980 and newer passenger vehicles. This oil has
superior anti-wear properties and enhanced oxidation stability over SE
lubricants.
SG: The SG rating was introduced in 1989 and combined the performance
properties of the commercial rating CC (lubricants designed for use in
supercharged/turbocharged diesel applications in moderate to severe
service).
SH: Now obsolete, SH was designed for 1996 and older engines.
SJ: Introduced in 1996, this rating is for all automotive engines
presently in use.
You can think of the API system as a blueprint for oil. In order to
gain an API classification, oil manufacturers have to follow a set of
limitations.
You can learn a lot more about oil at this link:
http://www.vtr.org/maintain/oil-overview.html
