Should I get an engine monitor & install an oil filter kit?
Should I install an engine monitor system and an oil filter kit to a Franklin 150 108-1? Do I need STC permission, or just throw them at my A&P/AI?
Sounds like you need a discussion with your A&P who is signing off your work.
On certified aircraft Maintenance repair and overhaul requires knowledge of FAR part 43 that details The Who what and how this is done. If you are asking someone to sign off your work for you and you don’t want to have your insurance invalidated you should be following the rules. A&P s are trained that this work requires information from acceptable sourness stating with the type certificate data sheet then ADs then maintenance manuals, parts cataloged, service information, AC 43.13 series and other advisories. All this comprises the approved and acceptable information mechanics are required to use while maintaining an aircraft. Everything a mechanic does has a piece of paper telling the what and how to do something.
Altering an aircraft requires acceptable or approved data. The oil filter requires approved data the engine monitor may or may not depending on the certification of the component. Some require an STC some don’t. The installation paper work should tell you. In either case both require a log book entry by an A&P, you need to talk to the person doing this. Sorry I’m a retired aircraft mechanic examiner, it’s easy to get into the weeds.
Yes you should get an oil filter kit for your engine. It's almost crazy to run an engine these days without some sort of filter. Back when you could ovhl an engine for a couple thousand bucks and Franklin parts were plenty, it wasn't such a big deal.
It really depends on your mechanic with the IA. Some would not touch anything that didn't already have an stc. Some would install an automotive remote filter mount, AN fittings, with a proper aircraft filter, and call it good. I have seen both set ups and the automotive set-up looks just like a Cessna 421 oil filter installation.