The OMC 5.5 and 7.5 horse outboards that era had counter weights riveted to the crankshaft to promote a smoother idle. This required part of the connecting rod be cut away to clear it. The bottom rod is trimmed on the bottom while the top rod is trimmed on top. The channel in the upper part of the connecting rod that collects oil and helps lubricate the rod bearings is missing on these top rods. The use of the four gallon tank lightens the load and also compensates for the less oil getting to the rod bearings. This is why both six and four gallon tanks take a quart of oil.
The newer oil has advantages. It lubricates better and burns cleaner. It also has disadvantages. It doesn’t seal the crankcase as well. Outboards without seals (like the 5.5 & 7.5 classics) can loose crankcase compression from worn bearings. Some antiques require SAE 40wt. oil for proper seating. Allso, the film left on the bearings evaporates over time which means a fogging oil must be used for long term storage.
