Honestly, any 10w-30 made for outboards today would be good. Mercury, Yamaha, Suzuki and other outboard manufacturers all have their own branded oils, but Valvoline, Pennzoil, Lucas, and other oil manufacturers also make 4-stroke outboard oils as well. Just make sure you get actual marine outboard oil, not automotive oil. Outboards run at higher RPMs than cars and trucks and the oils made specifically for outboards are formulated to resist thermal breakdown and shearing caused by running longer times at higher RPMs.
Also, you have an 18 year old motor that was run by someone else. Who knows how well they took care of it internally, so I wouldn't stress the small stuff. Personally, I'd just run Merc 90 High Performance Lower Unit Oil because that's what I keep around and is easily available year after year. Johnson/Evinrude products are continually disappearing and who knows how long that lower unit oil will still be available. But to each his own.