1248 Weeks in the top 10 according to my math.
"Muscles hung on in the top ten until 1976, 24 years after his first appearance on that list. He continued to enter tour events until 1980. At age 45, he won his first-round match at all three tournaments he played."
Why the Traditional Slice Backhand is messing you up - JeffSalzensteinTennis
He gives a good argument for variations of the slice backhand and the video is descriptive so I've included it. As a comment related to the video, here is my take on whether this is new or just used more often. I think also that the changing equipment has added to this option in style.
I learned the old-school slice backhand with a wood frame by watching Rosewall. I often used the "modern" style, as Jeff describes it for handling lower balls or choosing to add sidespin. Done to an extreme, I remember it as "peeling the orange, " and we'd use that on "body volleys" or an approach. I believe I've seen Rafa use that fairly often, while Fed used it for the same purpose among his many variations.
Here is a good demo that I found on Youtube if you want an example of a very smooth and effective looking slice backhand.