I use 10-30W oil in my Laycock Overdrive. I fill it using a pump oil can. Takes a while but it works for me. Fill it until oil comes out of the fill hole.
FWIW, to ensure a good ground, I run a ground wire from the unit to the frame. I do this because the unit started to shift in and out of overdrive on its own. I spoke to Rich Little, the guy who built my overdrive, and he told me to add a ground wire. It has worked perfectly since installing the ground wire.