2001 toyota celica loss of oil, dark chunky bits coming out of exhaust?

[chimp2dog]

my celica is spitting out what i believe is oil through my exhaust, i recently bought this car like 2-3 weeks ago and as soon as i bought it i did an oil change. i always noticed the dark bits splattering out my exhaust but i figured it was excess fuel from a bad injector, but i replaced the injector and that problem was fixed, but then my CEL came back on this time with a p0420. Catalyst System Efficiency Below Threshold (Bank 1). and with all that black gunk out of my exhaust i go to check my oil, and the dipstick wasn't getting oil, so i drained my oil to find that it was very dark and maybe only like a quart of it was there. i got lucky to catch this before it did any real damage. so i did some research and it seems as though the 00-02 celicas have this issue. so if oil is running through my exhaust system, could this be the cause of my p0420? if so how would i go about clearing that problem? running a cleaner through my cat? and about the oil it might be my rings but that seems way too expensive to fix. any help? i'm thinking about running some seafoam through it maybe it'll help? if the rings are stuck or something.