Quote Originally Posted by firebird77clone View Post
My 01 Silverado calls for 5-30 oil, it was low and I'll change oil in a day or two, I put in a quart of 20-50.
If you were just at the low mark of the dipstick, you could have easily went a couple more days without any problem. The danger with not enough oil is that the sump level isn't always high enough to keep air from getting sucked into the pickup.

Quote Originally Posted by firebird77clone View Post
Just what do the numbers mean?
As explained by others, the first part of the viscosity (20W) is the cold viscosity and it means that that your oil has the viscosity of a 20W-weight (W means Winter) oil. That is:
  • maximum Cranking Viscosity of 9500 cp @ -15°C and
  • maximum Pumping Viscosity of 60,000 cp @ -20°C

The second part of the viscosity (50) is the hot viscosity. It means that it has the viscosity of a 50-weight oil. That is,
  • Kinematic viscosity between 16.3 and <21.9 cSt @ 100°C and
  • HTHS of at least 3.7 cP
Your 20W-50 flows like a SAE 20W oil when it's cold and like a SAE 50 when it's hot. See SAE Viscosity Grades for Engine Oils for more information.

Quote Originally Posted by firebird77clone View Post
Would it damage the engine to run straight 20-50, as i have quite a bit.
Engines depend on flow for lubrication and pressure is characteristic of flow and viscosity. Using too thick of an oil means that the relief valve will end up recirculating more oil back to the sump. It's better to stick with the recommended viscosity but, if you really want to use up your stash of 20W-50, you could add 1 qt every oil change.