My spark plug is 8 years old, with 6k mi on it.Is it necessary to  change it if there is no problem?thanks!

I usually just check 'em and keep on running 'em until they start to give problems.  I try to also make sure that I have a spare new one at the ready.  No delays in case can't find one, all stores closed, etc.

After eight years you might want to pull it out and dab a little anti-sieze compound on the threads. Spark plugs that never get taken out of aluminum heads sometimes take the threads out of the head if they're left in too long. Not a catastrophe, but it can be a PITA to repair.