1979, would have been 20W50 almost for sure...the only one that you ever saw on TV was GTX 20W50.
All the service stations had their own home brands in pint and half quart glass bottles, but usually had a rack of Castrol GTX, and XL on the other side of the door.
BP Corse was 25W50, and had a following.
Mid 80s, Shell Super SF was 20W40, and they brought in 15W30 XMO, which flopped (would have been a great oil, being the start of XHVI).
Early/Mid 80s, things started to move into the ultra thick oils. If memory serves, it was STP that fist started with 25W60 and 70, an bizarely, the silliness took off from there.