Boarder Line Low Oil Pressure

the op made no mention of the motor having been rebuilt. neither he or the previous owner know any of the internal clearances. this tells me it hasn't been apart for quite some time. so any assumptions about a bad rebuild are just guessing and off topic. as some have said change the filter and swap to 20/50 before touching anything else. it could well just be a higher mileage motor that's getting a bit loose in it's old age and needs servicing accordingly, lol.
neil.