Wondering about using one of the many oil additives (Amsoil, for instance) to the existing oil in a stock 400 engine 68 GTO prior to doing an oil change. I am just now taking ownership of the car and really don't know anything about the service history on the car. Going to use Lucas oil with the zinc properties from now on, but thinking maybe a little extra TLC with getting the old existing oil out might be in order.....anybody have experience in this area or advice? I have pretty much always thought such additives aren't worth the $$$$ as long as there is regular scheduled maintenance, but in this case, I dont know what the maintenance has been. Thanks!!