I am looking for stocking up on spare spark plugs. But I get confused
as of which spark plug to use.

The spark plugs in the motor all have this marking:
BPZ8H-N 10 (note the 'Z')

The spark plug that SELOC Outboard Repair Manual (for my motor)
suggests using this spark plug:
BP8H-N 10 (note no 'Z')

Two local marine supply stores (West Marine, Boater World) only carry
this spark plug:
BP8H-N 10 (note no 'Z')

According to Spark Plug Application Guide, "BP8H-N 10" is for older
motor 1990-2000 (my motor) and "BPZ8H-N 10" is for newer motors
2001-200?. Seem like I should be using the older version to match my
old motor. But: If I start using an older version of the spark plug to
replace an existing spark plugs when it fails, I will be mixing old
with new spark plugs. Is this OK to mix old version with new?

Should I replace all the spark plugs with old version all at once?

Should I stick with the tried-and-true / new version of spark plugs
that are already in my motor? Where can I find this new version of
spark plugs anyway? Please note that I cannot find it in local store.

Please help. Thanks.

Jay Chan