I normally don't use all my boat seats, but have company coming this week so I took an unused one out of the garage. It has light black spots (mold?) where the seat (Ranger) was folded over. I tried ArmorAll Leather Care gel on them. They were lightened, but not totally removed. What is the best cleaner for leather that won't harm the seat?

http://www.myhorse.com/gear/tack/wipe_out_mold_with_leather_fungicide.aspx#top

I used StarBrite Mold and Mildew Cleaner - worked like a champ and didn't cause any discoloration.

Simple Green did a great job of getting mold spots off of my boat seats. -Hooch-

...but I always start with a shot of WD-40 on any stain- if it doesn't work, then go on to the stuff noted by the previous posts. I've also had reasonable luck on stubborn stains with the simple eraser on a number 2 pencil. If all else fails, try Comet Cleanser, but this may abrade the vinyl a bit. I have a few tan stains on my forward seat from the boat cover I use that uses the seat for some of it's support. I have found nothing that will touch these stains since the dye from the cover appears to have soaked into the vinyl.
Doug

If it is a factory seat......... it isn't leather.

The other posters gave some good answers.
I like simple green, too. Also you could try- 3M Vinyl cleaner...or Meguire's

Some elbow grease, a soft brush and some detergent is still probably best.
Don't get too carried away with abrasives.