I have a 1978 mercruiser 120 hp what all get gear oil and oil in the transmisson?

Not clear exactly what your question is, but I'll take a stab. The ';transmission'; in your outdrive is more often referred to as a ';gearbox';, located in the ';lower unit';. It requires a 90-weight heavy gear lube. Make sure you get a ';marine'; gear lube, which will do a better job of maintaining it's lubricating qualities in the presence of water.





Your lower unit has a drain plug located a little lower than the propshaft, and a vent plug above the antiventilation plate, both on the starboard side. They are easily recognizable because they both accept a large slotted screwdriver.





Remove the drain plug, then the vent plug. The drain plug should have a magnet - a small amount of metal filings is normal. Let some of the oil flow over your fingers, look for metal fragments. If you see metal in the oil, the gearcase needs to be disassembled for inspection.





Both plugs have plastic (nylon) washers; some people use new washers with every oil change, but they can be re-used if you don't crush them when you reinstall.





If the oil has a milky appearance, it means water has infiltrated the seals. It's possible the source of the water is the nylon vent/drain plug washers. You might choose to just replace the washers, and drain the gearcase again after the next outing. If you have water again, the lower unit seals need to be replaced. Some people might advise to disassemble and inspect the gearcase - I don't do this unless I have symptoms of bad shifting, or find metal in the oil.





When you buy your gear oil, look for a plastic pump (about $10, BassPro, Cabelas, Academy Sports, some Walmarts, any marine dealer will have 'em). Screw the plastic fitting into the drain opening at the bottom, and pump until oil comes out the top. Insert the vent plug, remove the pump and insert the drain plug.





By filling from the bottom, you ensure that the gearcase fills completely with no air pockets.





The plugs should be seated firmly and tightened, but there's no need to apply all your strength - crushing those washers just makes them more likely to fail.





Change your gear oil every year. I do mine in the fall before the first freeze; if any water has got in, it *will* crack your gearcase open when it freezes.