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Originally Posted by Seaver
The numbers are the thickness of the oil. In the oil commercials you hear about "viscosity breakdown", basically the oil looks it's stickyness factor. Thinner oils work better in cold environments, as it quickly circulates through the engine at startup to protect the metal-on-metal contacts. Thicker oils tend to take a bit longer (a minute or so) in very cold weather to warm up and go to the engine. That means, however, you're left momentarily with little oil in your engine. Once it's up and running, however, you'll be fine.
Thicker oils (10w40, 20w40) are much better at protecting engines over the long term and in hotter environments. The thicker oil means its viscosity will adhear better to the metal parts and stick more in the minute defection in the metals which provides more lubrication.
Personally I stick to the heavier stuff. Then again I live in the Texas heat, with a truck with a large V8, and have over 140k miles. So the lighter oils simply wouldn't hold up.
5w20 for you sounds extremely lightweight. While it may be in your owners manual, it just seems to me unless your engine is powered by a lawnmower engine you'll want something thicker. At next oil change I'd go with the 10w30 or 40. 10 weight is good for cars and works well in cold weather.
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Another thing about the weight that was not mentioned, is that in a multi-weight oil (one with 2 numbers like 10w30 or 5w20 etc.) the lower number is it's viscosity at low temperatures, such as the temperature when the car has cooled off and is not running, while the higher number is the weight of the oil when it is hot, during engine operation. You can find oils that have a very low cold weight, and very high hot weight (such as a 5w35 or 10w50), but they are touhger to find and much more expensive, and they don't work all that well in many passenger car engines due to the elastomer chains in the oil breaking down much easier, meaning more oil changes.
It isn't all too realavent, but just wanted to toss a bit of education in
