Trust me. Use B6ES plugs. There seem to work great for me for the last 3 years in the two RA28 18RG motors i owned. I'm still using them now, although at the moment i've got the Bosche equivelant in the car, its basically the same plug with a different name on it. They're pretty cheap, so they wont set u back a lot of money. And if u dont like them(no reason why u shouldn't) u can change back.
Up to u, but i always use B6ES, and i've done so for at least 10 or so sparky changes.