Oil for '48

Friend of mine has a '48 1/2 ton with the 216 that she wants to get back
on the road.  Last run about 10 years ago.

I'm wondering what oil to use in it.

Issues:

(1)  It has a glorified splash lubrication system for the rod bearings--
if the viscosity is too high they're not going to get lubricated right.

(2)  Flat tappets, not roller, so should have a good deal of ZDDP--I'd
rather waste 10 bucks on an additive than have to replace the cam.

(3)  The "book" says to use SAE 20 unless the temperature is going to be
sustained above 90.

(4)  Starting in 1955 Chevrolet was recommending 10w30 as an alternative
to 20 weight.

Most oils on the market today have had the ZDDP cut down from their
levesl 10 years ago to protect catalytic converters.

So the plan is to use a good 10w-30 with the addition of a zddp
additive.  I?m wondering if anyone knows of another alternative, or a
reason that that?s a bad idea.

I'm aware of "classic car" oils such as Castrol 15W-50.  Might be fine
for "classic cars? from the ?60s but I think it?s going to be too heavy
for the rod bearing lubrication system on this engine to work properly.

Anyone tempted to say "let the chemists do their job" must bear in mind
that the zinc limits are set by bureaucrats, not chemists, and the
chemists generally are not testing for compatibility with 60 year old
engines anyway.  And anyone tempted to say that they didn't have ZDDP in
1948 needs to do more research.