The type specified by the vehicle manufacturer in your owner's manual. For most passenger car and light truck gasoline engines today, it's any oil that meets the American Petroleum Institutes "SH" rating.
As for the viscosity of oil to use, most new engines today require a multiviscosity 5W-30 oil for all-round driving. The lighter 5W-30 oils contain friction reducing additives that help improve fuel economy, and also allow the oil to quickly reach critical upper valvetrain components when a cold engine is first started. Most engine wear occurs immediately after a cold start, so it's important to have oil that is thin enough to circulate easily -- especially at cold temperatures.
For older engines and ones that are driven at sustained highways speeds during hot weather, 10W-30 or 10W-40 is a good choice. Heavier multiviscosity oils such as 20W-40 are for high rpm, high-load applications primarily and are not recommended for cold weather driving.
Straight weight 30W and 40W oils aren't very popular anymore, but some diehards insist on using them. They say the thicker oil holds up better under high temperature (which it does), increases oil pressure and reduces oil consumption in high mileage engines. But straight 30W and 40W oils are too thick for cold weather and may make an engine hard to start. They may also be too thick to provide adequate start-up lubrication to critical upper valvetrain components during cold weather. So switching to a straight 20W oil would be necessary for cold weather driving. Straight 10W oil can also improve cold starting, but is very thin and should only be used in sub-zero climates. A multiviscosity 10W-30 or 10W-40 will provide the same cold starting benefits of a 10W oil and the high temperature protection of a 30W or 40W oil.
For the ultimate in high temperature protection, durability and all-round performance, synthetic oils are the way to go. Unfortunately, most synthetic oils cost up to three times as much as ordinary petroleum-based oils. They cost more because synthetics are manmade rather than refined from petroleum. But this improves their performance in virtually every aspect:
CAUTION: For vehicles under warranty, extending the normal change interval is not recommended because failing to follow the OEM's maintenance schedule can void your warranty.
Synthetics are available in the same grades as ordinary motor oils (5W-30, 5W-20 and 10W-30) as well as "extended" grades such as 15W-50 and even 5W-50.
There are also lower-cost synthetic "blends" that combine synthetic and petroleum-based oils in the same container. But you can do your own blend to save money by simply substituting a quart or two of synthetic oil for conventional oil when you change oil. Synthetics are compatible with conventional motor oils.
Who should use a synthetic oil? The premium-priced oil is best for: