Use the type of motor oil specified in your customer’s owners manual. Most manuals say it’s okay to use a variety of viscosity grades depending on temperature conditions. Generally speaking, the following holds true:
10W-30 is best for all engines for year-round driving. 10W-40 is more popular in the aftermarket, but 10W-30 is actually a better oil because the additive package in it holds up better over the long haul. This is why General Motors does not recommend 10W-40 motor oils for any of its cars.
5W-30 is approved for most late-model four-cylinder, V-6 and V-8 engines on a year-round basis. It is not approved for many turbocharged or diesel applications, some high output V-8s, or applications involving driving at sustained highway speeds or towing in hot weather. It may not be the best choice for older, high mileage engines.
5W-30 is used as the factory fill oil on most new cars because it pumps through the engine more quickly after start-up (important for keeping overhead cams properly lubed). It also makes cold weather starting easier and reduces fuel consumption.
- Straight viscosity oils have limited temperature ranges and lack the versatility of multiviscosity oils. They can be safely used as long as their temperature limits are observed.
- Straight 10W is okay for cold weather starting and driving, but too thin for warm weather driving.
- Straight 20W is okay for all around driving, but doesn’t provide the temperature protection of straight 30W (which gets too thick at low temperatures for easy cold starting).
- Straight 40W and 50W oils are primarily for heavy-duty applications.
Special multiviscosity oils such as 2OW-50 are typically formulated for racing or severe duty applications such as towing.
Synthetics are a good alternative for any of the above because most provide extended temperature protection and service life.