After 250k it's likely your O2 were probably past due for replacement. They don't last forever. Hard to say on your spark plugs w/o seeing them. How many miles, how much idling, how much highway speeds ..what kind of fuel??
The catalytic converters 'should' last longer, than 250k. Some people get way more out of catalytic converters, but hard to know what else could be going on. A leaking head gasket ...coolant really damages CC's. So, hard to say. Your cats could have conceivably been worn out anyway depending on factors we can't know. But more than likely they failed early (fouled) due to constant MMO oil going through them. A little bit of oil between long periods and the oil 'should' burn out of the cats. But putting any oil in the fuel tank, or diesel, etc routinely is like a vehicle that is 'burning oil'. And that does wear CC's prematurely.
There's really no reason to add anything to fuel. Just stick with Top Tier fuels and you'll be ok, unless there are other issues with a engine that need to be taken care of.
