General guidance from an BMW R90/6 but works similarily based on your specific models basic factory suggested version.

A NGK b8hs is fine if you live outside the city. If you have to negotiate lots of traffic lights on a cold engine I'd go for a b7 or even a b6. Go for a nice long run, then on an empty stretch of road slow down to a stop and immediately turn of the ignition, don't let the bike idle. Now take out the plugs (they are hot!) and have a look at the colour. Make sure they are grey/brown, not black (too cold, go down to a 6 or 7) or white (to hot, go up to a 7 or 8). Careful, if you keep on for too long with a plug that's too hot, you can damage the engine!

Usually the airheads have based on model the type specified as NGK example a BP7ES or BP6ES. The bikes before 1980 had usually higher compression rations, hence higher temperatures, and therefore the BP7ES is the right plug. Whereas newer after 1981 models have warmer plugs like BP6ES. But you can check your specifications below.

Spark plug type (Bosch)

W5D1: R50/5, 1974-1976 R60

W5D: 1977-1979 R60, R65LS, all 1977-1980 R100 models (R100, R100S. R100T, R100RS, R100RT).

W6D: R75/5, R75/7, R90/6, R90S, R100/7, 1981-1987 R100 models (R100, R100CS, R100RS, R100RT).

W7D: R80, R80/7, R80ST, R80G/S, R80RT

W7DC: 1988-on R100RS, R100RT, R100GS

Spark plug gap: Recommended 0,6-0,7mm (0,024-0,028 in.)

NOTE: BOSCH Spark Plugs are the correct NON-resistor spark plugs for Airhead models called for by BMW. They are only available usually from BMW dealers, because Bosch stopped distributing non-resistor plugs EXCEPT for the three types: W5DC, W6DC, W7DC, which they provide exclusively to BMW and are sold only with BMW 11-digit part numbers. Do not use Bosch WR5DC, WR6DC or WR7DC resistor plugs.

Similarily use NGK non-resistor models like BP7ES and not the BPR7ES that is a resistor spark plug.