Different viscosities for different temperatures?

[freshcedar]

So I just got a 1974 Dodge Tioga RV. Vinwiz tells me it's a Dodge B-300 1-Ton V8 360-2V (LA). Hard to find more info on it. I wonder if Mitchell and AllData have it.

So I guess I'm curious about where to find more information on these things, other than getting your hands on an original manual.

But I'm very curious about these different viscosities. What are the benefits of say a 20W-20 oil (WHICH I DON'T THINK EVEN EXISTS) as opposed to a 5W-40 or a 20W-50?

Maximum Performance SAE 10w30 Signature Series 100% Synthetic Motor Oil (ATMQT)
Performance Plus XL 10w30 Synthetic Motor Oil (XLTQT)
Performance Synthetic 10W30 OE Motor Oil (OETQT) 19 To 81 F......20W-20
Above 10 F......15W-40
Above 0 F......10W-30, 10W-40, 10W-50
Above 19 F......20W-50, 30
Below 61 F......5w30, 5W-40

 

[Astro14]

Oil was different in 1974, when your manual was written.

You can find a 20W20...but why bother? Pick a good 5W40 (or, in CA, 10w40 or 15W40) and be done with it...