Bought 05 GTI MK5 3 weeks ago, very happy, 1 owner, FVWSH, 59K miles. Driving home last night and oli light came on. Stopped, bought 1 lite of castrol for almost £20 (ouch) from BP petrol station (was late at night> Next day, light still on. Quick trip to trusted workshop and seems the sensor is the problem, apparently not uncommon for them, to go.
Lesson is, trust the (hard to read0 dipstick