i also agree with ransoman, if you have noticably more soot and worse mpg than other examples of the same model then you are overfueling.
apparently 'sooty tailpipes are normal'
think about it this way, you're combusting a carbon based fuel source, soot is a natural product of that combustion. how much soot will depend on the amounts of the different chemicals present during combustion. the amount of chemicals will depend on how compelte the combustion is, so your fueling is critical to the amount of soot produced.
the cylinder liners are cleaned by the engine oil (you can see the evidence of that by your black oil) however your valves wont get cleaned so soot can build up, causing a loss of compression. realistically that takes a very long time to have any effect, but it does happen. if you take a head off after 100k you can easily tell the difference compared to a head that has done 50k.
you can regularly add redex style detergents, or you can use super unleaded (it works out cheaper and more effective using redex), both will in theory lead to cleaner valves. in practice the effect it has is negligible because both redex and super unleaded additives are really just 95% parrafin. that parrafin can help to clean your injectors as it passed through them, but in the very dilute amount it wont really help significantly. it will clean the inlet valves as the fuel injector sprays pretty much on to the valve, but they are clean on that inlet side anyway becasue they are closed during combustion. it wont have any effect on your exahaust valves since the paraffin is combusted in the cylinder before it passes through the exhaust valve. the theory is that on the compression stoke both valves will be closed and the cylinder will then have high pressure parrafin cleaning on both valves. in reality this doesnt happen becasue it takes a lot more than a millisecond for parrafin to dissolve the carbon so it can be burnt, and the parrafin will be very diluted even if it did have more time to act.
to overcome the ineffectiveness of fuel additives you can give your valves a proper clean. remove the sparkplugs and pour some redex in each cylinder, then turn the engine over for 30 secs. now your valves are covered in paraffin, which dissolves the carbon deposits. wait 5 mins to allow that to happen, then refit the sprakplugs and fire it up so you can burn those carbon deposits off. i do this at every service.
pouring redex in your cylinders doesnt help with the carbon build up in your cat and the rest of your exhaust. what can you do about that? well unless you want to take your exhasut off and clean it with caustic soda (like you do on 2stroke bike exhausts) then there is little you can do. end of the day it isnt really a problem as it doesnt build up to a point where it resticts air flow, the only issue is if the cat gets coated so it is less able to reduce your emissions, leading to an MOT failure and the bill for a new cat. thats life.