OMC made three types of shifting systems, electro-matic, hydro-electric, hydro-assist. The electro-matic used a electro-matic coil that whould, when energized attract a coil spring, ground & wound in square section that wrapped around a knurled hub of the gear, " knurled on forward gear only" Thus, the more pressure put on the spring, the harder it would grab. You want a lighter oil with this system, as if the spring, and hub of the gear slip, they are done. The hydro-electric, and hydro- assist use the same gear pattern as used today. The difference in lube has to do with water temperature. The heavy lube will make these unit's sluggish to shift in cold water temps. as they are hydraulic. I use the newer HPF lube in my old 125 hydro-electric as I run it harder than most. Newer lube's in the hydro-electric, hydro-assist will work in warm temps. with out damage. They have the same roller, needle, and thrust bearing's used today.