Caballo said:
example: 0w-20
0= warmed up viscosity rating
w= "warmed up"
-20= cold or start up viscosity rating.
viscosity= how easily liquid flows. higher number means less flow potential/ more lubrication.
You need more lubrication at low temps/start up because of ring seal and emissions and friction. The faster the part moves the easier it needs to move. hence the lower warmed up rating. But if it's a HD part with a lot of friction or lubrication needs you need heavier weighted oil for example rear end gears,trannies, etc.
engine oil= 0w20/5w50
tranny/differential= 70w90/85w90(ish)
hope this helps
This is incorrect; the "w" actually indicates the oil meets viscocity standards to be used at 0 degrees F and thus qualifies as a "winter" oil. The first number is a measure of the viscocity of the oil at 0 deg. F. The latter number tells you how much the oil will thin. For example, a 5W-20 oil tells us that at 0 degrees, the oil has a vicocity rating of "5" and that at 212 deg F, the oil will not thin any more than what a 20 weight oil would thin at this temperature. Thus, a 0W-30 will flow even better at 0 degrees than the 5W-20, and it will not thin as much at higher operating temps as the 5W-30 oil.
See this link for a better explanation:
http://auto.howstuffworks.com/frame.../automotive/auto_technology/2002/8/motor_oil/
I run 5W-30 Mobil One synth in my car. Ford calls for 5W-20. This means that at full operating temperatures, my oil is slightly thicker than what the manufacturer recommends. Honestly, I'm not sure this is really what I should be doing either. If there are any lubrication points in the engine that could suffer from oil starvation from running a slightly thicker oil (thicker when at operating temps), then I guess I shouldn't be running the 5W-30. However, I'm going to guess that there isn't TOO much of a difference between the two oils (5W-30 and 5W-20) and since many, many folks run 5W-30, I doubt it's something I need to be concerned about. I would guess there is a marginal fuel mileage gain by using the 5W20 instead of the 5W30 that was recommended in the very same engine the year before. Multiply that per car marginal fuel mileage improvement across all the mustang V8s that they sell, and I'm sure it helps Ford's CAFE numbers more than just an immaterial amount.
Now, I'm asking myself - "Why do some folks run oils like '10W-50' in their relatively stock motors? Seems the 10W would be fine in the summer or warmer climates, but when that oil is heating up to operating temps, you are definintely running a thicker oil than what is recommended by Ford. I wonder if there's a more of a chance for oil starvation vs. the 5W-30 oil I'm using?
Or is it an immaterial differnence - kind of like I'm assuming the 5w-20 vs. 5w-30 is immaterial????
Interesting. . .
. Synthetics 
