po 420 and oil consumption

If I recall correctly, there was a recall for the 2002 2.5 liter that could effect the life of the motor (pre-cat?). You might want to contact a local dealer and see if it was done (they can look it up in a national database). If it had not been done (or done to late), I think you're probably well past the warrantee.

Gary K

Sad to say, it looks like he bought someones else's headache. They probably put a new head on it in an attempt to fix it. It likely needs a rebuilt block. How long ago did he buy it? I'd be looking into your state's lemon laws. The seller almost certainly knew. To answer your other question, yes, high oil consumption will shorten a converters life.

It isn't going to smoke because the converter burns it off..

Neither will cause oil consumption.

It's probably an internal engine problem. Most people who buy a "spec V" type car are going to run them hard. I've had to replace/rebuild a few of these with under 100,000 with bad rod bearings/oil burning. Loose rod bearing clearances can cause oil consumption like you describe. Try reving the car up and down fairly hard when cold and look for oil smoke before the cat can lite up. I bet it smokes, then the cat lites off and it stops.

Doubt a lemon law would apply to a used car. I also bet there was no pre-purchase inspection performed by the buyer?

Don't be so sure, MA for one has strong used car lemon laws. Even those from a private sale.