What oil to run in my rebuilt 340??

agree...need much more info. With that said...I'm a big fan of the Brad Penn stuff.

Most engines in good shape (tight-ish clearances) will maintain good pressure with 10w-30. That is what I use on my BB stroker and it has great pressure.

You may be able to run a lighter weight (ex 5w-20) but I think that may be pushing it for these older motors. Techincally if it can still build pressure, it should be fine but that oil is like water IMHO. For the few HP you may pick up on the lighter oil and the potential for losing pressure if the clearances open up any, I wouldn't go below 10w-30.

As for what brand, you will get a ton of opinions on that one here but you asked for weight only so I suggest 10w-30

Brad Penn. Lots of zinc.

Stock engine means flat tappet cam which will like/need zinc. Diesel engine oil like Shell Rotella T has it. It is available in 10W30 at Walmart. That's what I would start with. If hot oil pressure is low (below 20 lb) step it up to 15W40. There are lots of other choices but I like the convenience and reasonable cost of the diesel oil.

Note Mobile 1 in standard car grades does not have much zinc. The 20W50 does but is probably higher viscosity than what you need.

Another vote for brad penn.

My vote is Rotella T 10W40. I run the 15W40 cause it has lots of zinc for my flat tappet cam. They make a lighter oil for diesels in the artic that can be bought locally. I believe it is a 10W40 or 10W30.

Just my vote. And please dont run a fram oil filter, go Napa Gold brand.