At the moment I live at 6000ft asl in a climate ranging from 12C to 35C and my engine always runs cold (needle gets to the end of the white bar and stays there).

I've always had issues with fouling plugs so I got some NGK BP5ES.

Now I'm moving to sea level, temperatures from 20C to 38C and while I was there on my old BP6ES plugs the temperature gauge was in the middle.

Do I need a colder plug for that altitude and climate or is the ambient not relevant?