What weight oil should I use - Brand?

[38 coupe]

You have the correct fitting, it is the one the oil pressure sending unit is threaded into.
Your engine appears to be assembled out of several sources. Your car left the factory with a tangerine orange engine with EAB heads. The intake looks like a faded/worn version of tangerine orange. If everything is working I would leave well enough alone. Several times I have learned, the hard way, that if "it ain't broke, don't fix it".

[panic]

The SAE (Society of Automotive Engineers) viscosity (grade, not weight) is, by definition, thickness (resistance to flow). It’s the time in seconds for a given volume of oil to pass through a sized opening, not the oil’s lubrication properties.
A single SAE number (without the “W”), or 2nd number of two is the viscosity at high oil temperature: 212° F, or 100° C.
A number before the “W” (e.g., 10W30), or the 1st number of two (e.g., 10-30) is the winter rating (not “weight”), indicating the viscosity at low oil temperature: 23° F, or -5° C.
“Straight” (single viscosity) 40 grade not only reduces cold cranking speed, but gives no additional protection or safety to the crankshaft. It simply does not reach the bearings quickly at low temperatures, and does not flow through the bearings fast enough to cool them. Bearing (and crankshaft) failure is far more likely to result from too much viscosity than from too little, as long as oil pressure is acceptable at all engine speeds.
Cold 50 grade oil flows through a bearing as fast as asphalt flows through a drinking straw.
Multi-viscosity detergent oil provides oil pressure more quickly after start-up than single viscosity. The detergents and dispersants transport the dirt & debris into the oil filter for removal. “Mobil 1” 10W30 or 40 is excellent for all purposes.

[petehoovie]

The SAE (Society of Automotive Engineers) viscosity (grade, not weight) is, by definition, thickness (resistance to flow). It’s the time in seconds for a given volume of oil to pass through a sized opening, not the oil’s lubrication properties.
A single SAE number (without the “W”), or 2nd number of two is the viscosity at high oil temperature: 212° F, or 100° C.
A number before the “W” (e.g., 10W30), or the 1st number of two (e.g., 10-30) is the winter rating (not “weight”), indicating the viscosity at low oil temperature: 23° F, or -5° C.
“Straight” (single viscosity) 40 grade not only reduces cold cranking speed, but gives no additional protection or safety to the crankshaft. It simply does not reach the bearings quickly at low temperatures, and does not flow through the bearings fast enough to cool them. Bearing (and crankshaft) failure is far more likely to result from too much viscosity than from too little, as long as oil pressure is acceptable at all engine speeds.
Cold 50 grade oil flows through a bearing as fast as asphalt flows through a drinking straw.
Multi-viscosity detergent oil provides oil pressure more quickly after start-up than single viscosity. The detergents and dispersants transport the dirt & debris into the oil filter for removal. “Mobil 1” 10W30 or 40 is excellent for all purposes.

[jplutz]

Wow, I sure am getting an eddication about oil, all, erl...
Joisey Jack, I thought you were tugging my tassel with the 0W-20, til I walked into AutoZone! So many 0W oi!s!! I had to come back on line to see what that is about. New synthetics for small, hot, high rpm engines. So I guess you are using that on a fresh rebuild, or a "new" frenchy.
Anywho, I'm changing the "erl"(a Brooklyn friends term) shortly.
38Coupe, the pics don't do justice to the actual engine colors. The intake IS a very intense orange, not pale like the shots, and the heads are fire engine red.
Again, Thanks for all the info, and help, you too petehoovie, as always.
Yours Truly, JP Lutz

[Jack E/NJ]

Yep. Walmart's finest 0-20w synthetic. Idles 20psi cold, warm or hot. Revved 30psi. Early 38 21-studder refreshed 10+ yrs ago. Use it in all my stuff for ~4 years. A 300K 413, 05 Escape & 48 Case tractor. Before that 5W-20 synblends. And before that 10W-30.