I have the 7.4mpi and have the same soot problem heard the msd ignition does help, I fill up at the dock usually but sometimes on land. 89at the dock, 93on land. Never noticed a difference with either fuel and alot of the stations up here get there fuel from Green Bay, I just clean it off real quick when ever I have the boat on the trailer. Although our soot problem is minor if you compare it to my dads diesels.

In the Marine field usualy you can not hear detonation because of the water cooled exhaust, but when I put some Ft lauderdale petrol in my boat,I juiced it once or twice and heard a slight detonation, wow, this has never happened before.
cheap ass swamp gas I bought in Florida.
This was so called 93 Octane also. Yeah right!!.
My boat never labored before untill I hit the big
pond we call the Ocean, wow, I could loose 5 knots in a swell cruiseing to the Bahamas. But the trip back was cool and fast & intense.