The leather (if it is actually leather) on the 8x20 BL is so heavily processed it seems like plastic and is very resistant to water and fungus. I have some old optics with natural leather covering, and humidity is indeed an issue. The 8x20 BL I treat the same as any of my rubber/fake-leather covered optics and it still appears to be unaffected by water and humidity after over 10 years of regular use.
I have the Ultravid 8x20 BL (purchased in 2006) and the Monovid (purchased in 2009), which has much the same kind of leather covering. When I moved 18 months ago, the basement in my house had a humidity problem and the Monovid case developed a case of fungus infestation, whereas the Ultravid did not, nor did my Ultravid 8x42 BL (2008 vintage). Not sure why the difference, whether it is just random, but I wouldn't rely on leather treatment alone.