I use Amsoil synthetic...run it 100:1 as directed..lot less smoke and runs smoother..( mine is a 140 V4 Johnson thats a 50:1 with regular oil)..It seems expensive per bottle but works out near the same
Calvin, if I were you, I would stop running the 100:1 ratio on OMC products. There was a program on the late 80's to early 90's motors that they would inspect and replace the power heads if needed, remove the 100:1 ratio stickers on the sides of the motor and slap a 50:1 sticker in it's place.
The following is generally true: 1958 and prior used 20:1, 1959 thru 1963 used 24:1, 1964 and newer use 50:1. High performance usage (racing) requires a ratio of 25:1. New engine break-in (not DI) requires 25:1.
If it's a Maroon one better chase down the date of manufacture.
FYI 20: 1 is a quart to 5 gallons, 24:1, 6 gallons.
In 1957, the Johnson 15 was replaced by the famous 18 (the DC-3 of outboards). If your 15 is of that vintage (pre 1957), then run 24/25:1, which is approximately 6oz oil per gallon of gasoline.
A new low profile 15 came out about 1974. It was the same physical size as a 9.9. Those motors were 50:1 ratio, which is 2.75oz oil per gallon of gasoline.
100:1 was one of a few tragic mistakes by OMC in the 1980's. Don't go to 100:1 unless you do nothing but run your engine at idle speed.
It will not hurt a 50:1 engine to run it with a 24/25:1 ratio mix, i.e., twice the oil. Perhaps if you idle all the time, you might fowl up a plug, but the plugs should quickly "burn off" clean once the throttle is opened up for a few seconds.