straight from the horse's mouth (so to speak), do not use Lexol on shell cordovan (just use it on calf/cow leather):
"No Lexol! The Venetian is worth waiting for, and shell doesn’t need much in the way of products. A good horsehair brush and a soft, damp cloth will take care of them the majority of the time."
I have and still call bullshit on this. Nick gave absolutely no substantiation on to why Lexol would be bad, and it was just a passing remark. Also, DFWII is an advocate, from what I remember searching this exhaustively in the past, and dude seems highly knowledgeable.
I use Lexol on all my shell without issue. Some people also are against wax on shell, or advise sparingly. I use that, too. Maybe I'm just coo-coo. I also sno-sealed shell, and there's not really any reason to do that, lol. Maybe some detriment will show itself over time, but I have shell going on a year now that is a-OK.
I recommend people experiment and draw their own conclusions.