I never even heard of 5w20 until my Ford dealer starting pushing it down the customers throat somewhere around 2002...I don't ever recall seeing 5w20 on store shelves until 2002...The Ford dealer put pictures of the 5w20 everywhere in the service area and even handed everyone that came in to the service area a little booklet explaining how great the 5w20 oil was back when they first starting pushing it...So I assumed it was a brand new oil on the market.
I never ever heard of anyone using 5w20 in the 60's,70's 80's or even in the 90's..I don't think any quick lubes or dealers even had it in stock at that time as 5w30 was as low as you could go...In most places in South Florida 10w30 was as low as could go until somewhere in the 90's...Back in 99 when I had my 99 Grand Marquis a gas station made me pay extra to use 5w30.
I never ever heard of anyone using 5w20 in the 60's,70's 80's or even in the 90's..I don't think any quick lubes or dealers even had it in stock at that time as 5w30 was as low as you could go...In most places in South Florida 10w30 was as low as could go until somewhere in the 90's...Back in 99 when I had my 99 Grand Marquis a gas station made me pay extra to use 5w30.