I disagree...I've been using Slick 50 every 50,000 miles on all my cars since about 1990 or so...some have gone over 300,000 miles without a single internal engine wear issue...of course I'm very good about routine maintenance as well, so that alone is helpful.
I have used Lucas in some vehicles, most notably one place I worked was a Lucas distributor, and we ran it in all out fleet vehicles...and these trucks got the hell driven out of them and stood up to it well, but to really prove anything, we should have run one or two without as "control" subjects, for a true scientific appproach...
On my own vehicles I have been able to measure a very distinct difference in extenal engine surface temps (around 20-30F measured with infrared remote) with Slick 50 as opposed to without, that's good enough for me to accept that it must be reducing friction, as advertised...anyway I'm not changing my routine now, as it works so well for me...
I think Lucas probably does increase engine life in the long term, just due to increased surface film retention for cold starts. I sometimes wonder if it might work against you in terms of increased parasitic drag and lower fuel economy, but the effect would have to be very slight, I think...