That panel comes off pretty easily, just a bunch of screws to undo. My Jetta has similar sound deadening panel so I'm used to it. Once its off, oil change is really easy.
Regarding intervals, my '05 ST has 205,000 miles on it and I used Mobil 1 and a 10,000 mile interval. Burns no oil at all, oil even comes out pretty clean. On the 2012, I changed the oil the first time at 5,000 and am watching the level since it seems to be using some. Totally different from the '05 which does not use any at all over 10k mile interval. So far I have 3,000 miles after the change and it has used half a qt. which seems excessive to me.
2012 SE Hatch 5SP Sport
'05 Focus ST bone stock
'01 Focus ZX3- kids college car
Zetec powered F2000 racecar