Have an 05 Audi S4 (B6).
Previous owner said he used 0W ??, while the auto parts store said that for this model, you can use 0W or 5W and that you can interchange with no issues.
My understanding is that the lower the number before the W, the thinner the oil is.
Does it matter if I live near the Rocky Mountains (higher altitude) or anything?
I would like to just stick to one, which weight should I go with?
According to various sources (link 1, link 2, link 3) 0W oils are not specified to be used in VW 4.2 V8 40v (BBK) engines. Stick to 5W-30 or 5W-40 and you'll be fine. As a side note: the number before "W" mark corresponds to oil's viscosity at low (winter) temperatures, so it's only relevant to situations when the engine is cold, i.e. at startup.
The higher altitude doesn't really matter since the oil is pressurized in much higher order magnitudes inside the engine than the difference between high and low alt atmospheric pressure.
It can be found in your owners manual. You can also call a dealership and ask what it is. More precise machining and different functions of oil in more recent engines, It is always best to use the manufacturers recommended viscosity.
