Unless you have pinking, boosting the octane rating will do little. While the engine was designed for 98 octane, I never had problems with 95, and only put Super in for competition. And that was with a seriously looney crossflow, 1840, twin 45s and a ridiculous compression ratio.
Terraclean website is big on headlines and very thin on actual facts. Probably similar to Redex. Redex is basically a thin oil with a lot of detergents dissolved in it. Produces clouds of smoke and the feeling that must have done some good.
There is a problem with any liquid through the spark plug hole on that engine as a result of the design - flat head, bowl in piston. As a result, you raise the compression considerably and can create other problems as getting the bowl contents out is very difficult.
Put a new set of BP7ES in and go for a proper thrash! At least 20 miles, wait till water temperature hits normal, and increase the revs used.
BP7ES plugs are a bit cold for a standard engine, so I'll not rule out needing to revert to 6s but give it a go.
There is absolutely no point in fitting iridium/platinum plugs to that engine other than wallet emptying as no real performance advantage exists.
