With our strong sun in NZ, this is a common problem. I use a microfibre or very soft car cleaning towel very gently work on the "blocked" area, making sure to work into the natural grooves of the leather with Meguiar's Leather Cleaner. Then I leave it for a little while until it looks totally dry, then with another microfibre or very gentle cleaning towel apply the Meguiar's Gold Class Leather Conditioner.
Seats in my Z still look very good. Most people are surprised for an 04 how well the seats still look.
Good luck, and happy cleaning.
ps. Perhaps a seat cover might be a good idea for those summer days, then the sun tan lotion won't be a problem.
I made out of spa sized black bath towels, seat covers, drawing in the top to fit around the head rest with very wide elastic. Works really well, quickly sling on the cover when you're grubby with sunblock or you've been training or grubby mechanics in the car (because the covers they use are usually gross).