When I woke my CB750K2 up I removed all the spark plugs and squirted some engine oil in each cylinder, drained both the tank and the oil pan, filled tank with oil, then cranked the engine with kill switch set so it would not fire and observed oil pressure with a gauge to make sure it made oil pressure while cranking, which it did. Once I fired it up I ran a few heat cycles before I changed oil and filter. The initial oil I used for this was just some new Mobil 1 I had leftover from the Tacoma I no longer own.
There is a common Dorman auto engine oil drain plug you can buy at say Auto Zone to replace the right hand galley plug to measure oil pressure, you can just drill and tap it NPT for any old gauge suited to measure oil pressure, it is not suited to riding the bike though because it wanted to unscrew in my case anyway.
I found this with google which brought me right back here for the info.
1990-1998 Subaru 1.8, 2.2 and probably 2.5 liter engine oil drain plugs
here is an Ebay one ready to go actually, cheaper if you can wait the few days for it to show up
https://www.ebay.com/itm/153453008309But that fiber washer IMHO will want to come loose with engine vibration, did in my case anyway....good enough for a test crank and fire while you pay close attention though :-).
Then once you have run it a bit, have a fan blowing on front of engine if you just idle it in garage you can keep an eye on oil level for a few days and see if the drain back check valve leaks like mine did, not a tough fix as long as the piston is not seized in the pump like mine was. The Joker Machine oil pressure gauge is pretty cool but only if you actually want an oil pressure gauge onboard forever. I had a 1/8NPT gauge kicking around but another $10 will get you one from Ebay, the temporary setup can be put away in your tool box in case you ever need it again. Good to know that you have GOOD pressure before you light the fire tho IMHO :-).
Bill
