Hi Red91,
I have never used this LM oil, but I have used lots of 20W-50 oil in cars of that vintage. I live in a warm climate, so they were fine, no problems.
In the Australian Commodore cars they used a SuperCharged 3.8L V6 from Buick. In Oz the owners manual said to use 20W-50 or 15W-40, in the USA I'm pretty sure you guys were using 10W-30 etc in the exact same Buick engine.
My problem with a lot of common 20W-50 oils around me, is that they are made to older oil specs like API SG, and tend to have a low TBN ~6, so I see them as six month oils, made to a low price point.
So I like the look of the LM 20W-50 which is made to more moder oils specs like API SL and ACEA A3/B4. It looks like a modern quality interpretation of 20W-50 that is aimed at the high end of the market, not the low end like a cheap SG 20W-50. If it's the most up to date A3/B4 spec it should have a TBN of 10 or more, if it's the older A3/B4 spec it should still have a TBN of 8 or more, like many modern oils. So I think this LM 20W-50 should be more of a 12 month oil (given reasonable miles).
If I needed a modern 20W-50, then yes, I would happily use this LM oil.