What weight oil should I use - Brand?

[jplutz]

Greetings & Salutations,
I just got this beauty back on the road after rebuilding 95% of the brake system. When I changed the oil & filter, I put in 10w30 cuz that's what I had. At idle, cold, the pressure is great, as per photo. And driving around the neighborhood it still stays good. But..The other day in 90deg heat I took her out on the local highway for a 2 mile round trip drive, and by the time I was returning home the pressure was in the pits, unless I reved the engine and raced down the road.
I realize that I screwed up with the oil. What weight should be used? I have a gallon jug of 5w40. Is that ok, or is the 5 side too lite? Is there a brand that is best? BTW, the engine did not overheat, the temp was steady.
Thanking you in advance, I remain
Yours Truly, JP Lutz
ps. I have dog dish on one side & baby moons on the other, and I paid thru the nose for the patina.. ha, ha!!