There are two basic types of OP gauge- mechanical & electrical. The mechanical ones involve running an oil line from the engine up into the cabin where the gauge measures it directly. Probably the most accurate kind, but they are a hassle to install and a disaster to clean up from if something leaks. You don't usually put them in anything but race cars.
The electrical ones have a sensor that converts the reading to electricity and then sends the info up a wire. You get way more flexibility on where to mount it.
For my money, VDO makes the best ones, but there are numerous quality brands out there. The usual hassle is figuring out where to mount it. I have yet to install one in my own subie so I don't have advice for you there. For my year/model I appear to have the option of getting an OEM model but it's very expensive.
Keep in mind that even with the gauge, you may not be able to detect bypass condition in the filter. It's obvious in some cars and not in others. If you really, really care about your oil it's a nice thing to have anyway.
I bought Mann brand filters from rock auto. I've used them on lots of other cars, came to trust them. Previously I was using the blue OEM Subaru ones, and I was buying them from a local indie shop for about $6.50.
Regarding OCI mileage... if any of your cars are still in warranty, change it as often as the manual states to protect your warranty. I think this is 7500 miles but read your manual to make sure.
After the warranty is up, get a used oil analysis and see if they say it could run longer. Then make the judgement call. If you run it 7500 miles and the lab says the oil is still in good shape, maybe try 10,000. The longer you run it, the more you save. If you aren't into checking the level between changes, then extended runs are not for you- don't try it.
You can use the lab report to tell you if you should consider changing the filter in between oil changes. At the end of the day, it's your engine and your checkbook.
