The D16 is the right plug, (or the D21 for slow runnng), D9 is not recommended.
We used to have problems with oiling up and lead in the fuel as well, not any more. Today I only sell D16's, use one in my TSEFPC, years of service with them no problem.
Years ago I also stocked the A6 now no longer available but replacement AB6 is, though I have not found them at the right price tto buy as yet.
Strang thing is the D16 is made in the States, yet I have many customers from there sending to me in the UK for them!
One of my 40+'s had a nearly new 8 com in it when I got it, so I left it in there and put on many hours trolling for bass, but also with 3/4 throttle dashes to and from. Out of curiosity, I tried a D16 for a few days and could detect no difference whatsoever. The 8 coms' ground electrode lays beside the center wire whereas the D16's hovers over it. The heat ranges seem similar, and at 25:1 with full synthetic and synthetic blend oils, I've yet to foul a plug. The 40 + at least, seems not to give a damn which of these plugs it uses.
And yes, the Champion D16 and D21 are readily available at most auto parts stores for around $2.00 US.