NGK's recommendation for Rapides, Shadows, and Lightnings (and the single equivalents) is the BP6 - I have an obsolete NGK plugs booklet which gives recommendations for every old bike known to man. Grade 6 plugs are recommended for virtually everything. However if your plugs are oily, and you don't want a rebore and new valveguides just yet, a twin will run all day on the hotter BP5, which is hot enough to burn the oil off. (I've since bitten the bullet so have reverted to 6's again.
Caveat: I've been using NGK since 1974 with no problems. In the last 5 years I've had 5 fail, 4 of them in the last 2 years, all BP6 type, and all four almost new - mostly 2 or 300 miles. The last one was yesterday, after about 50 miles. It still has shiny plating on the thread.