The last LA smallblock I did was a few years back....it had low oil pressure when I bought it as a rebuilder. I had issues with a new cam I bought. It was a new billet ground by a local company. The prob was the new billet had far narrower journals than the original cam. Combine that with a little core shift in the block and it was far enough out to barely cover the rather large oil holes in one of the cam journals. From memory the hole was like .020" inside the bearing....it was THAT bad! I have no doubt if I ran it like it was,or had old cam bearings in it there would be oil pressure issues. I ended up getting an old original cam reground. I have no doubt there are still billets floating around like the one I had. The 273 was a solid cam engine from memory? I'm pretty sure the oil system is similar to the Ford Clevelands,fed via lifter bores and all other places before we give a little oil to the ummmm,what are they?? Oh yes,the bearings...maybe they should get the left over oil !! My point is,if the lifter bores are worn,then you run used cam bearings that may be a tad loose it's not surprising you have low idle pressure. From memory the oil pressure is read from the rear,but that's where the pump is,so actual pressure at the front of the engine may be even lower. I'd feel a little uneasy about that idle pressure,sure it may be within limits,but a new engine should have solid pressure about 10lbs below normal cruising pressure in my book. As far as oils go,I can't really suggest anything for your climates,but If you have large clearances with that thin oil,it's going to show up any issues sooner ,but that being said,if it's a good engine you should be able to run that viscosity without hassles. In my book bearing clearances you measured are fine... a little tight if anything. Full grooved bearings are not on my shopping list and may also be costing you idle pressure,there is no advantage to them. There are many,many guys in OZ running 500+++ HP Clevos with CHI heads turning 7000rpm and they never run full grooved mains...they dropped that idea with cross drilled cranks,it's old theory...and the Cleveland isn't known for its state of the art lube systems!i feel the LA engines would be in the same boat..If you need a crank ground it's usually better to measure the ID of your bearings and get the crank ground to the clearance you want,.001" per inch plus.0005" is a good yardstick. If the STP is helping,then maybe a heavier oil could boost things a little,but it's not really a fix,but it won't hurt ... I've used STP in engines that need oil pressure to tension the timing chains,it helps shut them up at idle...sure replacing the chain is the fix but it certainly extended the time before I did the chain service. I looked at a rod bearing when it was torn down and it was perfect,no issue,a new chain and guides and all was good,so I don't feel it would hurt....but you shouldn't need it.
To sum up all my rambling I think I'd be checking lifter bore wear,cam bearing condition,cam journal width and alignment,go to half groove bearings and check oil pump RV..the fact you have had issues with 2 different pumps tends to indicate that's not the issue,but check....the other two alternatives are to..run it as is,or run heavier oil...what's to lose? If the oil doesn't fix it you'll pull it down later rather than sooner,it's really just a matter of preference. Less than 20 at idle and 50 at high rpm and I think I'd be stripping it. And yes,I used the "F" word (Ford) on here..
