Your reservoir is you lube level reference. The shaft is about half submerged at vent level with drive down. After the drive warms up a few times most of the air burps out into the reservoir and the shaft is almost fully submerged.
Your safest bet is to buy a lube pump and two - three bottles of Merc High Performance Gear Lube (blue stuff) and an oil collection pan, and get your boat on a trailer to change the lube and have extra to keep the reservoir topped off.
To change your lube, trim the drive down as far as possible and still get the pan under the bullet nose. Remove the reservoir cap, then the drain plug on the bottom of the drive. When drained, remove the upper vent screw, attach the lube pump and lube bottle into the drain hole and pump until lube spills from the vent. This will take more than one quart, so you'll have to switch to the second bottle of lube. When lube spills from the vent, insert the vent screw and seal washer and keep pumping until you see fresh lube start to fill the reservoir bottle. Top off the reservoir and replace the cap tightly, then remove the lube pump hose and quickly replace the drain plug and seal washer.
If you can't arrange this, keep you reservoir topped off. I've resealed shot seal, milky oil outdrives and was surprised to see the critical gears and bearings in good shape. But don't push you luck. Try to get fresh lube in the drive, or get it fixed ASAP.
