I have been using WalMart brand SuperTech and Mercury Quicksilver oil in my '01 200HP Yamaha HPDI and was thinking of switching to Penzoil Synthetic Blend.  I still have about a gallon or so of oil left in my tank and was wondering if anyone knew if I can just top off the tank with the synthetic blend, or do I need to flush the old oil out of the system first.  I have heard that if you mix the synthetic blend with the regluar oil, there may be a problem with the two not being compatable and actually gelling up.  Anyone have any experience with this?

TJ

If I was going to switch, I would dump the tank.  Dumping the tank should be done every couple of years anyway.  Almost every one I've seen seems to accumulate water in them any way.  I've seen more than one motor blown because the tank had so much water it sucked it into the oiling system.

I've never heard of it causing a problem mixing them, I've done it tons of time premixing the gas/oil in the gas tank, but I always take the tank out, dump it and clean the screen when I change oils.  

As a safety precaution, anytime I do anything that opens the oiling system for the posibility of and air pocket, I add enough oil the the tank to premix the gas in the tank to a 50:1 ratio.  This double oils the motor for that period of time but it insures an air bubble doesn't get hung up in the pump and cause a no oil situation in the motor.

Thanks for the reply.  Are you saying that I should mix a little oil in the gas tank?  I thought that was a big no-no on the HPDI, or am I misunderstanding something.

TJ

Sorry, not a big no no, just a big waste since gas does not go through the crankcase in the HPDI to lube the the crank.  Kinda read over the HPDI part.  In your case, just make sure you get it bled out good if you dump and clean the tank.