Answer: I understand that Aprilia owners are having problems with oil changes these days. The Rotax engine has a dry sump... Wet sump is more logical and easy to understand. The oil in a wet sump is right there in the bottom of the engine for the pressure pump to use, the rider can see the level in a sight glass, or the bike has a dip stick like a car right on the engine block... The first real Honda motorcycle I ever owned had a dry sump. The seller pulled the cap off and showed me the oil pumping into the bottom of the tank, and said that as long as I saw oil pumping into the tank, I hadn't hurt the engine... I think he was trying to justify himself, the dry sump was almost empty, but I didn't understand that, and never added the required two quarts of oil, and burned up the engine in three weeks... In a wet sump, the oil just sits there in the bottom of the crankcase or the oil pan, and it slops from side to side as you turn to the right or left, or slops up and down as you go over bumps. The oil may actually interfere with the rotation of the crankshaft, the power lost is called "windage" loss... Some early designs had no oil pump, and actually used oil splash to lubricate the bearings, maybe using a dipper instead of a pump to pick up the oil and fling it around. Like Chevrolet six-cylinder engines that had no oil pump from the 1930's to the 1950's... My old 1949 Whizzer motorbike had a dipper blade that flang oil everywhere, but the rod bearing was toast... Later engine designs had pressurized lubrication systems and even had sheet metal windage trays to turn the bottom of the crankcase (or oil pan) into a tank underneath the engine. Such engines with deep oil pans (or crankcases) are taller than dry sump engines and the center of gravity of a wet sump motorcycle may be higher than a dry sump motorcycle, but engineers have found that a rather high center of gravity assists the rider in making the bike turn in rapidly... A wet sump bike that turns in rapidly is not as stable as, say, a Harley Davidson Big Twin with a dry sump thundering along in a straight line, with little interest in changing direction rapidly... While cornering (or doing aerobatic maneuvers), oil may be forced away from the pressure pumps pickup tube and the engine may suffer from air entrained in the oil or even pump cavitation... Dry sumps require two oil pumps, a pressure pump and a scavenging pump to return the oil to the tank, so you have two pumps that might fail instead of one. Dry sump engine probably has a check valve to keep the oil from the sump draining back into the bottom of the engine. Check valve doesn't always seal properly... And you have two sets of lines to have air bubbles in... A separate, sheet metal tank on the side of the bike gets hot, radiating heat from the oil, perhaps to the rider's inner thigh (or maybe his GF's inner thigh)... But the location of the wet sump, a foot or so above the pressure pump inlet *theoretically* ensures a supply of oil to that pump... 'Tain't necessarily so, if the scavenge pump falls to pump the oil *up* to the dry sump, there may be little or no oil at the pressure pump inlet... Piston-engined airplanes often have dry sumps. I remember flying across country in a radial-engined T-29 transport. After only about 1000 miles, we landed in the middle of Texas and the mechanics pumped 40 gallons of oil into the dry sump tank on top of the engine nacelle...