Quote Originally Posted by Joe Camilleri View Post
You will notice on the 20w50 Valvoline racing oil that it says "Off road use only".
Phosphorus tends to poison catalytic converters so they don't work so well. Hence reducing phosphorus additives to prolong catalytic converter life, i.e. "prolong the life of emissions control devices." With the move towards catalytic converters that last over 100k miles, reducing phosphorus poisoning is important.

For a similar reason, to prolong the life of emissions control devices, new diesel oil also has reduced ash content. Ash is the stuff that accumulates in a diesel particulate filter and doesn't burn off. This is why a particulate filter has to be cleaned out every 120-150k miles. Best analogy is to think of a wood stove. The stuff left over, that doesn't burn, is the ash. So over the life of a vehicle, reduced ash in the oil means reduced ash into the exhaust, which means reduced ash stuck in the filter.