Gonna go from cheap oil to Pennzoil Synthetic.

[depfreeman450]

Bought boat new in 2001. Been running just whatever kind of oil in boat since then but decided to start taking extra precautions and start running a better oil. I am going to start with Pennzoil syn. because it is more readily available in my area. Can I just add the synthetic to the oil that is already in the oil container/pump in back of the boat or do I need to empty the oil that is in there completely out. What is everyones opinion on penzoil syn. ?

[Mr Limpet]

Mix it in and run it. I use it in everything and never had a problem. I've never noticed a performance gain from any oil brand, for that matter. Talk to a motor rebuilder for the gains if any are associated with a particular oil.

I do know that decarbonizing your motor is more important than your choice of oil.