Yes,all you have to do is loosen the cap and let it idle till it is
overflowing. Tighten cap, you're done. But if visual inspection shows the
reservoir not to be full, fill it then do the idle thing, that will save
time as the oil just slowly fills the reservoir. I guess the internal
pressure could or does help supply the oil injection pump but so does
gravity. I don't know what would happen if you ran the engine at operating
speed , without oil system pressure . Now one BIG issue. DO NOT premix as a
substitute for oil injection, your engine will blow up, this is not a maybe
it WILL. Reason is the Optimax as well as other direct injection engines (
Yamaha HPDI and Evinrude Ficht) have the fuel injected into the combustion
chamber, is does not go thru the crankcase, so premix will only lubricate
the piston and cylinder walls, no oil will ever get to the main bearings and
rod bearing and case seals. No matter what you are told don't believe it, it
just ain't so.
As far as why oil could have got low in the reservoir , it could have a
cracked cap, cracked tank or leaking hose or even a leak of pressure at the
main oil tank or maybe even the pressure pulse to the remote tank is missing
( bad check valve leaking hose or cracked oil tank or that cap may be loose
or cracked), other than the caps these are not so common but are all
possibilities. So don't overlook carefully checking out the whole oil system
to see that it is pressuring up. Your idea on the sender is good, to
troubleshoot that too, disconnect it and if it is still alarming then short
between the sender wires, either open circuit or closed will be the
condition the computer is looking for to say, things are ok. You have a good
engine and this is a simple problem for now so get it fixed right.
I hope this helps, good luck,

Ron White
Boat building web address is
www.concentric.net/~knotreel