OVERKILL
$100 Site Donor 2021
20W-20 was typically a monograde (SAE 20) with a decent enough base oil and PPD's to help it meet the 20W Winter grade requirements. HTHS would have been around 2.7-2.8cP which is probably on-par to what a 10W-30 would have ended up at after the VII's very quickly sheared, due to the quality of them during that time period.20W-20?
This is part of the reason Mobil had no problem recommending Mobil 1 (which was a 5W-20) for applications that called for an xW-30 back then, because it would have held its viscosity considerably better than heavier multigrades of the period.