My choice with my 2020 tech update is as follows, first what the book says OEM standard plug NGK C7E, Champion G59C OEM racing plug (cooler) NGK C8E My thoughts are to install the NGK C8EIX now that we have the Iridium tech in our 2020 pocket I will be running mostly Shell pump premium (ethanol @ 10% here in SoCal) with some brand differences by need basis. The book calls out non-resistor plugs so non-resistor plugs it will be.
Bearing in mind the 610's have the single cam head vs the dual cam head on the 630, my experience with the dual cam heads is that the engines respond really well to anything that improves combustion. For a while I had a 2010 TE510 together with my TE and Sm 630 and all of the just loved iridium NGK C8EIX plugs I recently came by some octane booster which I added to 95 RON gas we have here and both 630's responded really really well to that.
Well its firm, CR8EIX it is that is what is available here. All my inter-tube research shows not much difference. But I do want to go with the cooler 8 heat range "for racing" plug
Most of the 610's will were carbureted whereas the 630's were fuel injected ( as was my 510 ) with not the best EFI either and I think that is why the iridium plugs on the EFI motors smoothed out the fueling and allowed more fuel to be usefully added via the JD tuner.
