I talked to a guy at a refinery about this yesterday. Ash is not harmful to a gas engine at all if you have no converter. It will kill the cat. converter. If it has no converter, it is not a problem at all. Chevron Delo 400 has 1.35% and Valvoline race oil has 1.5%. My Boss uses Valvoline race 40w in his boat and has over 1500 hrs on it with no problems.
There is also something in the "motor oil bible" about how sometimes high ash numbers can be decieving, as some detergent additives contain metallic derivatives which are common sources of ash. They also mention that because the ash is left behind as the oil burns, if that oil has a low NOACK value, less ash will be left behind as well. So an oil with a high ash content, but low NOACK value, is probably better than one with a low ash content, but a very high NOACK.
I'd use the ACEA numbers as a guideline for now, but a lot has still got to be learned. Much of the ash comes from the Detergent, but the more detergent, the less damage from higher zinc and phos. I'm using Delo in my Grand Cherokee (gasoline).