The catalytic converter on my Prius gets clogged with oil and performance suffers. Is there an oil that will result in less issues with the CAT? I read someplace, that phosphorus and sulphated ash content that is particularly damaging to the cat. Would an oil carrying a rating such as ACEA C1/C2/C3, or a dexos1 or ILSAC GF5 for example help in my situation? It looks like the ratings with the lowest sulfated ash content would be: ACEA C1, lowest SAPS (Sulphated Ash, Phosphorous, Sulphur) or ACEA C4, having low SAPS but a higher HTHS rating. How does API FA-4 rating stacks up? In this 2007 prius with 260,000, I use Walmart Super Tech full synthetic high mileage 5w-30 (& whatever filter is handy) and change at 4,000 miles. I tried a can of CataClean and it made a world of difference, better mileage (back to 49mpg), significantly improved acceleration and best of all the exhaust fumes stayed out of the car (not sure how a clogged cat sends exhaust into the cabin?). Sadly the improvement was short lived and after about a week I was right back to the square one. Other things I may try; replace PCV valve, replace spark plugs, install an oil catch can, an engine flush, use Techron fuel additive for a month, baby the gas pedal, clean the mass air flow sensor, try a higher viscosity oil 10w-40.... If I could just keep the CAT from being clogged I would be all set.