"some mineral oil to keep the pump mechanism seals lubricated and happy"
That could be a very expensive habit. Mineral oil is petroleum oil, and the fine for dumping ANY oil overboard...should be located on your oil discharge placard. (Second & smaller fine for not having one of those if you are 26'+ OAL.) In many districts, if the USCG or any water constable sees ANY rainbow slick coming out of your boat--you've bought it. And, there's a toll-free EPA number that anyone else can call.
So, ecology and ethics questions aside...oil in the head can be very expensive, unless you're Real Damn Sure you'll only be using pumpout stations.
And you'd probably get better results from dumping the same amount of ArmorAll or other water-soluble "green" plasticizer in there instead. Mineral oil isn't really going to stick around, or rejuvenate rubber or leaether seals in water.