Just an FYI, I've done cylinder drop tests on Yamaha HPDI's in neutral at an idle.....and they do idle on all 6 cylinders.
I think what you may be confusing that with is the "shift interrupter" which momentarily kills the spark on 3 cylinders when initially shifting into gear, and out of gear, in order to make the shifting process go a little easier. (So that there's no hard CLUNK! when shifting).
Many Mercury V6 outboards are similarly designed.
Mixing extra oil in the fuel will trick the ECM into thinking that the injector pulse width is too rich....and will limit performance.
And finally "Ring Free" , while it doesn't hurt anything, does literally NOTHING to help your engine.....it is just yet another hit to your wallet. Save your money and use it to buy another Wiggle Wart. π
Nothing jack's up a HPDI engines operating system any worse than mixing up some Grandpa invented witches brew in the fuel tank. Everybody needs to stop doing that !
Now, let's review.....What goes in the "FUEL.TANK" of a DFI outboard?
That's right.....FUEL ! Nothing else. π