2 months ago I purchased a new Bayliner Capri 160 BR, with a 2003 90hp merc OB. When I picked up the boat I bought 1 gal. of oil from the dealer, Mercury premium 2 stoke oil. I now need some more should I keep using this oil or should I switch to something less expensive? I have seen the other post which recommend penzoil. If I do switch oils do I need to drain the oil that is in the resivoir? Does anybody know where to pick up a repair manual for a 2003 90hp Mercury outboard I can only find up to 2000. Thanks for all the help in advance.