We were running D-16 on our 0-540, like kb said, same gap, we would put about 80 -100 hours on a set of plugs. Would swap top plugs to the bottom after two oil changes. This helps alittle if the rings are loose and you burn alittle oil. Doing oil changes every 25 hours. Good plug, should not have any problems.
We switched to Denso L14-U (or #5000) a while back. Same gap 0.024-0.026. They seemed to foul less if you let the boat idle alot (which we do). Main reason for switch is that the base seems rusts alittle less than Champions around salty conditions. Last time I had both Champion and Denso plugs in at the same time. Hard to find the Denso in some places.
Lycoming has some front engine mount oil filter adapters and some adapters that are mounted on the firewall. However a Lycoming filter runs between 15 and 25 bucks - OUCH. We got the adapter for hose that run to the firewall, purchased a good hydraulic filter adapter (25 buck) and get good hydraulic filters for about 3 bucks. Get some hydraulic hoses made and you are set up for 40-50 bucks. Have the oil go from the engine to the cooler to the filter and back into the engine. Lots of the time the temperatures sit around 140 while at cruise. Still changing the oil and filter at 25 hours.
Aircraft operators go alot longer between oil changes, however at few things to remember. They are not throttle jockeys, they set the throttle at one RPM and run for hours. We are revving those motors up and down, which produces more wear and tear (so I was told) on the engine. They typically have a air filter and are running through relatively clean air. Lots of us just have a flame arrestor and no real air filter, some are running bottom intakes and sucking any small bits of dust, dirt, and sand that gets into our boats into the chambers of the motor. I would either go with frequent oil changes like kb or add a filter, and still wouldn't go very long between changes.
kb - you might be overkill, but I bet you will get some hours on that engine. That is one department where overkill is not bad.
Wally - one last tip, use the silver never-seize on the threads of the spark plugs. Otherwise you will end up putting in heli-coils. Learned that the hard way.