Recently I wanted to buy a new spark plug for my WD m20.
I always had a NGK plug, B6HS (and nr 4510 written on the box).
The guy at the shop only had B8HS (and 5510 written on the box).

Would this be OK to use ?

Why experiment just because the bloke hasn't got your plug in stock......Plenty of other shops/ebay/internet.

Personally I don't like NGK. I used Champion L86Y for about 30 years but now seem to get better results with Bosch W8AC. buy some of each quite cheaply here.
https://www.gsparkplug.com/?gclid=CjwKCAjw7cGUBhA9EiwArBAvojwX0gvavDliY576NS9sBxmBjhYlG_DfgZx_meVyJkqsb9LsIbB4yRoCHhQQAvD_BwE

However I am now running two of my regular bikes on NOS original 1940's KLG plugs that acquired from USA and Denmark.....As an experiment!

Ron

email (option): ronpier@talk21.com

B8HS will be ok if you ride faster ?? sorry

email (option): pes.sales@btconnect.com

any 8 NGK is two grades colder than the 6 you are running
If you do long rides they may be fine but on short rides thy will tend to carbon foul
They will also tend to do this on the over run going down long hills under engine breaking .
The 4 character number underneath is a stock number and of no consequence to you if you regap the plug ( which we all should do )
The same plug with different gaps set at the factory get a different number as does the same plug in blister packs and in factory 144 plug tray .

Plain B anything is being deleted in favour of
BP , projected electrode which is what I use
BPR / BRP which are both resistor plugs and should be avoided as they put a high load on the magneto .

As adviced, get some 6's from some where else
FWI I use 8's 7's & 6's for different times of the year & different types of use .