When you run the engine at full throttle you will get higher pressure even from splash systems, the faster the liquid moves the higher the pressure, a liquid compress a bit, but less than air. The wheel creates pressure on liquid and air, the highest pressure is on the solid moving parts as they strike each other. I rembember changing a gasket on an old 4 stroke mower that had oil pressure behind it. The advice for modern engines is to get going ASAP. I buy small amounts of fuel. Water contamination is a also a problem with all sorts of liquids, fuels, and lubricants, especially brake fluid. My uncle used to have a cheap green small chain saw and the fuel would boil in it after a brief period of work and the engine would stall. Stihl has a new 4 stroke that uses a fuel oil mix. I have a mulcher that has a two stroke Briggs and Straton engine. My sister used to have a B&S Rover and had all sorts of problems with it. But it was badly maintained. I have a problem with a governor on a B&S, I think the spring to the carby from the control arm on the governer is over strecthed, I have not tried to fix it yet.