The solenoid works just fine on 6V, like stated above. The current draw is small maybe a few amps. If you here it click, that means the plunger is moving. As said above, drain the oil and install what you fill comfortable with. People have used syn gear lube, motor oil, that what I use, 10W-40. The O/D will work in all 3 gears. With switch on, start in first slowly, should feel it shift into O/D, meaning the pump built up pressure to move the cone clutch. Mine was bult by Rich Little, he makes a great product.
You can take the bottom plate off, when you drain the oil and look at the magnetic filter plate and clean it.The oil should come out clean.