The problem is with leaving plugs to long is that with the relatively high temperatures involved with fork lift engines they have a tendency to seize into aluminium cylinder heads. We have a number of trucks come in p/ex which have this problem due to the plugs being left to long, we had a Linde in just this week with this problem! For the very low cost of spark plugs these days its not worth mucking about removing and copper greasing old ones, so we just fit a new set. We also tend to fit new cap and rotor on a service, and over the past few years this has shown up on our records to have saved us a lot of money on call outs for ignition failures. Nowadays these are also very low cost parts and not worth leaving to chance.