How to change lower unit oil in johnson 90 horse outboard

1) remove the lower drain plug and place container underneath
2) remove top filler plug and allow to drain out bottom
3) pump new oil in from the bottom untill it overflows out the top
4) replace top plug
5) remove filler hose from bottom, and quickly replace bottom plug

Tip: keep in mind the amount of oil that should fit, it should specify in the owners manual. If less than that actually goes in it could mean there is an air bubble, if this is hte case, rotate the prop until the air bubble leaves and then continue pumping in the bottom.

ALWAYS REPEAT THIS AT LEAST EVERY YEAR, PERFERABLY EVERY 100 HOURS.

Source: boat mechanic.