First of all, oil is the substance that lubricates the internal moving components of your vehicles engine. There are many different types of oils on the market suitable for car engines but there are some basics that you should know when purchasing an oil.

Viscosity

This is simply how readily the oil flows. A higher viscosity oil will withstand more heat than a lower viscosity oil. However, at colder temperatures it can actually become so thick that your starter cannot turn over the engine - and worse yet it won't flow to all the moving parts it should. A viscosity that is too low won't maintain a lubricating film on engine parts - allowing them to make metal-to-metal contact. Either extreme is very bad!

SINGLE GRADE VS MULTI-GRADE

Single-grade engine oils (such as SAE 30) used to be the standard for engines operating at high temperatures (such as air-cooled engines). Since the advent of the American Petroleum Institute (API) rating system, the multi-grade oils that proliferate shelves now-days are suitable for nearly all applications. I did have an older gentleman tell me today that his '30 Ford won't build oil pressure with multi-grade oil - but then again, I'd be surprised if his '30ish Ford builds oil pressure at all....

Coincidentally, SAE stands for Society of Automotive Engineers.

SELECTING YOUR GRADE

So which oil you should choose will mostly depend on what you're driving. If your car is not an antique or air-cooled, the safest bet is to use a multi-grade oil. The first number on a multi-grade oil indicates the "cold" viscosity of the lubricant - the second number the "hot" viscosity. For example, a SAE 10W-40 motor oil would have a viscosity rating of 10 at 0-degrees Fahrenheit (the W indicates "Winter") and a 40 rating at its maximum recommended temperature (not engine operating temperature - ambient air temperature).

The following chart is a good place to start. Determine the temperatures you'll be driving in and select an oil accordingly.

Also consider your driving habits and the condition of your car. If you drive hard - keeping the engine near redline for extended periods of time - or you do a lot of high RPM "freeway" driving - then you may want to select a higher viscosity oil for your car. Also, if your engine is high mileage, a high viscosity oil will keep it a little quieter and may not slip through the rings to be burned (blue smoke).

If your car is newer, you will most likely want a lower viscosity oil. This is true of most newer high-performance cars also - they too have tighter bearing tolerances and recommend a low viscosity oil (it doesn't rob as much power from the engine).

Over-all, If you expect very cold weather ahead, definitely choose an oil with a lower "cold" viscosity. If you live in a warmer region, a higher winter rating (the W rating) will help keep your cam and valves from clattering when you first start your car.

I hope this helped. Please - remember to Vote and vote often....