OP: My Focus is remapped, so you could argue it needs servicing more often. A few years ago it did 6000 miles in 18 months (between services), but I did this knowing I am generally very anal over my car servicing. Decent parts, good brand of oil.
How 'good' your oil is depends on lots of factors. If you have just a small amount of water then the lubricating effect of the oil is reduced, plus it degrades more quickly.
I'm no diesel lover, but the 2.0 136 has a 6 speed box and is generally a reliable lump and is quick enough. You shouldn't have a DPF either.
Keep it until:
1) It struggles to start (injectors can't build enough pressure. Replacement = £££), but don't confuse this with it struggling to start as low battery voltage or engine has just been sat for ages!
2) You gently touch the clutch and can feel violent shaking in the pedal (DMF replacement = £££. Add a clutch too).