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Yes, it is amazing to me that Honda offers NOTHING about checking the trim/tilt fluid in their owner's manuals. I'm guessing they made a corporate decision that the trim/tilt units are "no user serviceable components inside"...including the fluid! Pretty ridiculous in my mind.
Anyway, back to your question.
They do list the "manual" tilt valve and instruction on how to use it in most of the manuals I've seen so you should have that information if you have the owner's manual. If you need to download a free one, go to:
http://marine.honda.com/owners/manuals
And select your specific outboard.
. This can sometimes be difficult depending on the transom of your boat and if you have any accessories in the way like swim steps or trim tabs. Also, you will need to rig a device to add fluid if needed because it has to go into the hole horizontally.Right next to the manual tilt valve, will be another slot headed plug. That is the fill/check plug for the tilt hydraulic fluid. You are supposed to check the fluid level with all the cylinders collapsed
Yes, you are probably correct that you have low fluid or air in the system. The problem here is that you need ask yourself "where did the fluid go?" or, "how did air get in there?". Because these units don't "use fluid" and are supposed to be water tight but they will leak.
As far as what to put in there, it's going to depend on the year the outboard was manufactured. But, generally you may use either a clear, light, universal hydraulic fluid or ATF. Dexron or Mercon is a good choice as well as 10w20wt tractor hydraulic oil. Both are sold at Pep Boys or Auto Zone or No Ability No Assistance (NAPA). Know way? NO HOW! (IMHO).
The trick is to approximate what ever is already in there. It is not a good practice to mix the two types of fluid although it can be done in an emergency. Rule of thumb is; clear fluid came out, hydraulic oil...red fluid...Dexron II, III, or IV. I would avoid using Dexron V for the time being as it is supposed to be a more exotic formula than it's predecessors but I'm guessing it will work too in a pinch.
Good luck.