I was wondering if there would be any reason not to add an oil tank and have an oil resovoir for the gearcase of an outboard. I have a 250 johnson. The main advantage would be having it vented so that pressure and vacuum could not occur within it. Oil level could easily be checked. Is there any problem with having the gearcase completely full of oil. Currently there is an air space to allow for expension but with the tank the oil could expand into it. I just replace my gearcase for 4 grand do to water getting in and do not want to do it again.