Engine Oil Viscosity & ZDDP

"10W-40" means: viscosity behavior of an SAE 10 oil when cold (W = winter), and an SAE 40 oil at operating temperature (100°C / 212°F). The lower the W number, the better cold-cranking performance. The higher the second number, the thicker the oil when hot.

Recommended Viscosity by Ambient Temperature

Lowest Expected Ambient	Common Recommendations
Below −20°F (−29°C)	0W-30, 5W-30 synthetic
−20 to 0°F (−29 to −18°C)	5W-30, 5W-40
0 to 30°F (−18 to −1°C)	10W-30, 10W-40
30 to 60°F (−1 to 16°C)	10W-30, 10W-40, 15W-40

⚠ Flat-Tappet Cam Warning (CRITICAL for pre-1985 engines)

Modern API SN/SP/SQ oils contain dramatically reduced ZDDP (zinc dialkyl-dithio-phosphate) — the anti-wear additive that protects flat-tappet camshafts and lifters. Modern levels are around 600–800 ppm of phosphorus; flat-tappet engines need 1,200–1,400 ppm minimum.
Running a stock 1965 SBC, Mopar B-block, or any other flat-tappet engine on Walmart 5W-30 will wipe a cam lobe in as little as a few hundred miles. The damage is silent until you notice a misfire or pull a valve cover and find a flattened lifter.
Use one of: oils labeled "Hot Rod / Racing / Classic" (e.g. Valvoline VR1, Brad Penn, Driven HR / XP series, Lucas Hot Rod & Classic, Joe Gibbs HR1/HR2, Amsoil Z-Rod); diesel-rated oils with the API "CK-4" or older "CJ-4" designation (15W-40 Rotella T4, Delo, Delvac) which still contain higher ZDDP; or modern oil plus a separate ZDDP additive (Comp Cams, Lucas, Driven) at every change.
Roller-cam engines do NOT need elevated ZDDP — the rolling contact does not generate the boundary-friction conditions that need it. Stock LS, LT, modern Hemi, modern Ford modular: standard API SP oil is fine.