Oil viscosity - why the 20W-50 BS?

Oil viscosity - why the 20W-50 BS?

My '72 Coronet is a hair away from 200,000 on its original , untouched 318 ; 70,000 of those miles are from my use .

I've been running Valvoline 4-Stroke 10w-40 (e.g. , wet-clutch motor cycle oil with ZDDP) and Lucas Oil Stabiliser in the colder months (usually Oct.-Apr. here) and 15w-40 / Lucas in the warmer months.

20w-50 is not recommended in 273's & 318's (more-so 318's) in factory-stock configuration , as the distributor shaft and oil pump drive are prone to snapping due to being strained by thick oils . This was especially a problem in the 1970-1972 engines due to some casting flaw (or so I've heard for the last 20+ years ... ).

So long as you're using a ZDDP-rich oil on flat-tappet cam engines , use whatever weight you desire ...

... just play keep-away from Pennzoil and Quaker State unless you want waxed-up rockers and a plugged oil pickup :D