From our experience, the VAC techs recommend these specific resistor type NGK Spark plugs for M50, M52 and S50us / S52us that are stock, or mildly modified with bolt ons.
The heat range of a plug should be matched to the engines used to ensure proper spark plug thermal performance. If the heat range is not optimal, then fouling of the plug or at the other end of the spectrum- detonation can occur.
The optimal firing-end temp is approximately between 930°F and 1475°F and if the temperature varies too high or low from these temps carbon fouling or plug over heating (and possibly plug failure or pre-ignition) can be the result.

In our testing and years of use, these factory original replacement type plugs work excellent on both stock and lightly modified engines.
Despite the claims of other "fancy" types of plugs, these have shown the best performance on the Dyno as well as a smoother idle and plenty of lifespan.

Under hard performance driving conditions, checking spark plug gap and/or replacing these plugs on a regular 25-35k mile intervals is recommended for best results.

We always keep these in stock for whenever you need them!

NGK BKR6EK Plug shown in images, OE for E36 M3, this plug works well in all E36 6-cylinder applications

Fits:
-M50 engine
-M52 engine
-S50 engine (M3 US version)
-S52 engine (M3 US version)