The internal combustion engine is not particularly efficient. Typically a gas powered engine is about ~60% efficient and a diesel is ~85%. The higher the compression ratio, better efficiency, so a gas engine at 10:1 compression ratio is less efficient than a diesel at 20:1 CR. Plus of course diesel fuel produces a bigger bang for the buck.
So if 60% of the combustion ends up producing power (HP), the rest is lost to heat escaping, (exhaust), and mechanical friction losses. Which leads us to today's topic, Engine oils.
If you buy a jug of oil, on the back in a little circle, you will see something like"SAE 5W20, API SN, GL-5"
What's all that mean? SAE (The Society of Automotive Engineers) certifies the oil's viscosity as 5W20. What is viscosity? Viscosity is a liquid's resistence to pouring. Water has a viscosity of 1 and is the benchmark. So 5W20 starts out as 5 and slowly gains resistence as it warms up until it is a "20 weight."
Hydrocarbon or crude oil based, "Dino" oils must have additives to alter the viscosity, whereas Synthetic oils are formulated to react to heat in a stable transition from start to fully heated. Dino oils have irregular molecules that break down over time, while synthetic oils have virtually identical molecules that remain stable and pourable from as low as -50 F to +400 F. Both types get "dirty" over time and need to be changed. Don't use diesel specific oil in gas engines. Diesel oils have more detergents and diesel additives.
The API or American Petroleum Institute monitors the manufacturers requirements for oil's wear properties, additives etc. and issues ratings. Gas engine oils started out at SA and are now at SN. GL-5 is a European rating similar to the API and is often referenced for German vehicles. Diesel is listed as a C rating, up to CJ currently.
One has to be careful when purchasing "Synthetic" oils. There is no such thing as a fully synthetic oil (marketing name), since most synthetics have a maximum of ~30% base stock. A company can call an oil synthetic as long as it has been a chemically modified petroleum base stock. If you look at the MSDS for a particular oil and it lists Hydrocarbons, Parafins, or modified petroleum as the base stock it is not, to my mind "true" synthetic oil.
A number of years ago Mobil 1 sued Castrol over that exact point but lost when Castrol proved that their base stock was sufficiently modified to qualify as synthetic. The chief advantage of synthetic lubricants is pourability when cold, which protects your engine on start up and improves fuel economy during warm up..
In my opinion a "true" synthetic should have a base stock of something like Ester oils, Poly Alpha Olifins, plus other additives, including zinc compounds. There are many types of synthetics on the market, some are even purple or red. The colour has no bearing on the formula. All the major refiners are producing good synthetic lubricants.
My personal first choice is Exxon's Mobil 1. I don't agree with their former tanker captain hiring practices, but they make a good product, since 1973, and stand behind their lubricants
So buy the correct viscosity for your vehicle, use a quality filter, and change as required.
As always, your questions, comments, and likes are welcome.