Generally, I like B8ES NGK's for my A10's and A65's. For me, Champion's always foul out.
More'n a dozen years ago, I ran accross a bargain on ND plugs, W20ES-U, and W22ES-U, and I got several boxes at the time. They've been working fine for me all this time. I still have a few boxes left. Looks like I have plug stock for a few years yet...
Loc: Sydney Australia
As a general rule modern "fuels" require one grade hotter than specified for real petrol. The more time you spend running slowly or at idle the hotter plug you will need. This is simply to prevent the heavy oilly gunk in the fuel condensing out on your plug causing it to foul. On a club run I go 2 grades hotter while n long trips I usually go only 1 grade hotter which in the M20 means a BP5HS or BP6HS. The actual equivalent plug would be 1/2 way between a B7 & B8.
Check if you can run the projecting electrodes without fouling on your piston by putting a lump of plasticine around .040" to .050" on the end of the plug & kicking the motor over.
Platinimum or any other "exotic" metal plugs will foul less than standard plugs as standard plugs were designed to burn stuff called "petrol" which sadly is no longer available.
I use N4c in my '71, though I have also used B7ES. Zelda seems to like the Champions just a little bit better. I also have the Sparx EI. I have used 5K resistor caps, but it gets a fatter spark with non-resistor caps. I currently use NGK model LZFH non-resistor caps with copper-core wires.
As I understand it, resistor caps are only needed with a digital EI or digital regulator (or if you listen to the radio while riding. )
A good friend will bail you out of jail. A great friend will be in the cell with you saying, "Damn, that was fun!"