I've seen similar accumulations of cleaner inside cabinets on other components.
I did a quick spray cleaning of the controls with some radio shack cleaner/lubricant gunk without having to open the thing up.
It's only been running now for a few days, and the problem seems to be diminishing as I treat it, but I will probably pull it and open it up for a checkout at some point soon if things do not clear up completely.
I'm determined to get this Yamaha receiver running cleanly upon start up because once it gets warmed up and settled, it really sounds quite wonderful matched with my Triangle Titus speakers and M&K sub in my 2 channel A/V system.
I don't recall my Dual 1264 with Goldring MM cartridge ever sounding better, even with my old Tandberg TR2080, which is a very highly regarded vintage receiver. I hadn't been inclined to use it much for years given my main system...so far I'm finding it to be a most pleasant surprise given the marginal cost of acquiring the Yamaha. CAble TV and DVD sound is also at least as good as I've heard on this setup with the Yamaha.
I also still have a late 80's vintage Yamaha cassette deck that has served well without issue when called upon over the years.