Luis,
All 4-stroke motors have some blow-by gases.
In the case of land-based motors, once up to operating temperatures, the crankcase temperatures are high enough to keep them vaporized. 50-some years ago, these gases were vented by a road draft tube, depositing crankcase vapor (including oil droplets) on the roads. After about 1960, the road draft tube system was replaced by the PCV (positive crankcase ventilation) system, which draws those fumes into the intake manifold, resulting in them being burned.
In the case of outboards, crankcase temperatures are typically much lower, even though the combustion chamber temperatures are about the same as their land-based counterparts. Because of that, the fumes condense into liquids, resulting in a gain of fluid in the crankcase. This applies to all outboards from all manufacturers. 4-stroke outboards usually have a crankcase vent tube that ends near the intake, so vapors get drawn into the motor and burned. However, the condensed fumes still accumulate in the oil, resulting in "gaining" oil.
Oil gain is worst with high-viscosity oils, with synthetic oils, and at colder temperatures. To minimize (not eliminate) oil gain, run NMMA-certified type FC-W, non-synthetic oil, the lightest viscosity that the manufacturer recommends, and run the motor hot and hard. Prolonged idling contributes to oil gain. Operating in cold water also makes the oil gain worse.
