Your boat may be the second largest investment in your life. If the Johnson outboard motor that powers the boat is large enough---it may weigh between 300 and 500 pounds---the boat has a hydraulic power trim/tilt system that tilts the motor to give you optimum performance. The type of hydraulic fluid you use can have a direct effect on the trim/tilt system's operation.
Hydraulic Fluids
The manufacturer's recommendation is, of course, to replace lost power trim/tilt fluid with the product produced under license from General Motors and sold under the Evinrude name, Evinrude/Johnson Power Trim/Tilt and Power Steering Fluid. In the event that the fluid marketed by the manufacturer is unavailable, Evinrude makes a specific recommendation to replace the fluid with GM Dexron II automatic transmission fluid.
OEM or Recommended Fluid
Hydraulic fluids are based on mineral oil. They transfer force between surfaces and transfer heat generated by compression away from the hydraulic surfaces---the pistons and cylinder walls that make up the hydraulic ram in a power tilt/trim system. Ideally, a hydraulic fluid will have sealing capabilities and will act as a lubricant. The best hydraulic fluids won't separate into their component chemicals---a process called demulsification. When hydraulic fluid demulsifies, it will not provide a uniform transfer of heat and force between hydraulic surfaces.
Other Hydraulic Oil
Not all hydraulic oil is compatible with the Johnson power tilt/trim system. Since Johnson specifies the Dexron II automatic transmission fluid, however, any automatic transmission fluid that carries the Dexron IIE, Dexron III or Dexron VI name, available from GM automobile dealers is acceptable---the newer Dexron hydraulic fluids are compatible with applications that used older versions of the product. If an Evinrude dealer or marine supplier isn't available or doesn't carry the Evinrude product, you can purchase the equivalent at a local GM dealer.
Changing the Hydraulic Fluid
You should check the hydraulic reservoir every time you use the boat. When you notice that the hydraulic fluid isn't quite up to the neck of the reservoir, top the fluid off; don't let air get into the system because it can do more damage than the lack of fluids. When you find yourself adding fluid more than once a season, it's time to overhaul the hydraulic system. When you take O-rings out of your boat's power trim system hydraulic system, don't re-use the O-rings, replace them. If they were leaking, they're already damaged beyond repair. Finally, when you overhaul the system, invest in a new hydraulic fluid.