API Standards
On every package of oil sold in North America an API service code appears on the container. This donut shaped symbol is made up of three parts that will help us identify the application and performance level the oil has achieved.

The first letter is either an S for spark ignition or a C for compression ignition (diesel)

Next will be a letter that refers to the standard the oil has met.
M is the newest standard for gas

I-4 is the newest for diesel

Keep in mind that it is possible for an oil to pass standards for both gas and diesel.

In the center of the symbol is the oil viscosity. The Society of Automotive Engineers (SAE) established viscosity. The higher the number rating the thicker the oil is. Viscosity is defined as the time fluid takes to flow through a standard sized orifice. The longer it takes the higher the number. As oil gets thinner as it gets hotter testing is done at established temperatures 212deg.F and 32deg.F or 100deg.C and 0deg.C. Single grade oil is tested at just 212deg.F. It still gets thinner and thicker. The manufacturer just tests the oil at one temperature to cut costs. Multigrade oil uses viscosity modifiers to enhance oil flow when cold while remaining thick enough when hot. 10w/40 for example (w = winter) flows like 10 weight at 32 deg.F (0 deg.C) and 40 weight at 212 deg.F (10 deg.C)

Synthetic oil
In the 1920’s two German scientists named Frans Fischer and Hans Tropsch developed a process of extracting a liquid fuel from coal to produce the first synthetic fuel. During World War II this method was further refined to produce lubricating oils for crude oil starved Germany. By 1944 between 10-20% of all oils in Germany were synthetic. By the end of the war German scientists were responsible for the preparation of 3500 different esters including diester and polyol ester.

The first commercially available synthetic oil was manufactured by French oil company Motul in 1971. Amsoil introduced one in 1972 and Mobil 1 in 1974.

Advantages
  • better high/low temperature viscosity performance
  • improved chemical and shear stability
  • resistance to oxidation, sludge and thermal breakdown.
  • reduced deposits
  • in some cases extended drain intervals
  • improved fuel economy due to decreased friction
Disadvantages
  • cost; about 3x regular oil or more
  • not suitable for break-in due to low friction
  • increases cracking of plastic components Which oil should I use? For owners of non-synthetic equipped cars a few things need to be considered i.e. type of use
    • a low mileage, seldom driven car would benefit from synthetic
    • extreme use (autocross) would benefit from synthetic
    • desire for longer drain intervals; the car may benefit from synthetic but the owner should consult their technician to determine if this is suitable for their engine type i.e.
      *whether engine is naturally sludgy
      *has rope seals or plastic valve seals
      as any one of these factors makes it unsuitable for long drain intervals.
    For the rest of us the recommended oil with the highest API standard (SM for gasoline engines) is the only one to consider and it should be changed every 5000 km or 3-5 months.

    Synthetic oil change intervals should be no more than 10,000km or 1 year as only the base of the oil is different not the additive package which still expires at approximately 5000km.