Oil has Five distinct jobs. It Lubricates, Cleans, Cools, Seals and Deadens Noise. Basic lawnmower engines have "Dipper Rods", they literally scoop the oil and force it into the bearings. They don't have oil pumps. Having oil sloshing around the crankcase of a conventional engine doesn't lubricate anything. It's actually detrimental to the performance of the engine. The film of oil between the bearings and the journals provide the lubrication that's required. Any oil splashing around the outside of the crank/rods doesn't "find" it's way into crank journals. It's physically impossible for any oil to creep into a gap that's oil a few thousandths wide, especially while it's spinning. As soon as the engine starts, there is oil pressure that's forcing oil through all the passages. In a wet sump application, whatever oil is in the sump doesn't provide anything except a source of oil. You don't want oil slopping around the crank while it's running, that's why performance engines have Windage Trays. It keeps the oil from the sump from splashing on the crankshaft. The dry sump system is a better design because it keeps the crankshaft from running in oil, which is the entire purpose. The return oil being the source of lubrication for the rocker shafts is minimal. It's not subject to direct oil pressure, but return oil pressure which takes longer to generate. This is not a real issue because the design does work. They may not have oil pressure immediately after start up, but a 10 or 15 second delay is not a problem. At idle the rocker shafts don't see a tremendous load and they do have an existing oil film. Add to that they have a very limited range of motion. They aren't spinning around like a crank or connecting rod. The oil has a tendency to stay put. As long as there is a healthy film of oil on the bearing surface, there is no problem. The rest of the overhead could run almost dry. I'm sure you've seen the pictures of the antique engines that run without valve covers and have an exposed overhead. BSA engines were designed to run with enough oil in the sump to supply the return pump and distribute some oil to the rockers. Theoretically as soon as the run-off oil hits the bottom of the sump. it's gone. I used to see 6-8 ounces of oil in the sump after a few days. Over time, it has gotten better and sometimes it may only be four ounces. I may be able to wait longer before draining the sump, but it's just a habit. I remember when I was young, my father would let the bike sit and he'd put a baking pan under the motor. When the engine started, there was oil everywhere. At least I can prevent that now.
