I've seen this in Huckleberry too. Cap 5 to 30 cm in diameter, light to dark brown with faint rings, chalky stem, gills bleed latexy stuff, grows under oak. I'm guessing some kind of Lactarius, but I've yet to bust out Demystified. LOTS of these things out there. Like, lots.
Ok, after walking through the Lactarius key in DM, I think this might be the Golden Milk Cap (Lactarius alnicola). I didn't taste it for bitterness, but it seems to match all the other criteria, especially it's abundance under oak.
Observed near in Briones Regional Park, near Lafayette, CA.