Re: "MOTOR CONTROL SYSTEM FAILURE" changed my life
the black on the inside of the exhaust is normal on a heathy car. if you rub your fingers in it, it should be very similar to soot powder, its carbon. its the same black stuff you get if you run a lighter on a knife blade.
it does sound like coil failure imo, especially considering the previous oil contamination of them. i always buy coils at a scrapyard, never more than £10 each, and very very easy to fit yourself. literally a couple of bolts.
you can also diagnose your coils using a multimeter. check the resistance between the 2 pins on the connector, each coil should have a very similar resistance, and that are 'significantly different' should be changed even if the engine appears to run fine. even ignition ensures even running which gives good fuel economy and performance.