Biased as it sounds to some, theres two things I don't like to run in my cars based off experience. Platinum plugs, or multi-ground plugs. Call me old school, but the engine feels more alive and pulls harder when back to basics. I know I'm a new guy here at M42club, but heres my cents:)
M42 cars, I run standard copper core autolite (3923) single electrode plugs. Yes the cheapest plug usually available at any auto store for the M42. Gap ranges depending on summer or winter. Widest I set is .036" gap. I also like magnecor KV85 wire sets for this engine. M42 cars at the moment are stock. The 318is will eventually have mild cams and chip.
Autolite makes a racing series plug for this engine but the AR series is one heat range too cold, or 2 heat ranges too hot variants. Also there's non-resistor available but it's one heat range too cold. Might be hot ticket for boosted setups, but NA stock engine the car didn't like them so much even in summer heat.
S14, ngk copper core, single electrode non-resistor, .040" gap. Bosch red coil. magnecor KV85 wire sets for ease of availability. Internally this engine is stock.
M10, autolite copper core, single cut back electrode, non-resistor AR52 .050" gap, Bosch red coil, accel 8.8 stainless core custom made wire set. This engine isn't stock.