I use 98 and full synth motorbike oil and have had no further issues with carbon deposits. I have noticed slow improvement, but its probably best you remove them.
Rip the head off, plug the transfer port with rag and use some small files (rats tail, etc) to clean out the port. Make sure no loose carbon falls into the cylinder, and don't be too hard and score anything
If you can track down a cleaner like caltex techron with PEA cleaner in it, then find some thick grass and mix 10 ish ml into a litre of 25:1, find some thick grass and give it a bit of a hammering, that might work as well
I think Nulon pro strength extreme clean has it - caltex dont sell techron here any more
