Can I use a spark plug with a heat rating of 6 for an engine with a heat rating of 8?

Hi guys, I have a 102cc Pollini engine and it's fitted with a spark plug with a heat value of 8 and a diode. Now I have another spark plug with a heat value of 6 and three diodes. My question is, can I use this spark plug? Here are the links to the engine…

How many miles do spark plugs last on a 500cc motorcycle? Do they make any difference?

The service booklet states that the spark plugs should be replaced after 12,000 km. How many more kilometers will the (2) spark plugs (regular, not iridium) last? Do the spark plugs make themselves noticeable in any way before they break? Or is there any way to determine the condition or "still shelf life" without removing…