I have a 14 yr old car with very high mileage as I'm doing a lot of commuting at the moment and don't fancy throwing money at a newer car just to drive it in to the ground. It's petrol and I change the oil and filters every 4 - 5 thousand miles, using synthetic oil which works out around £50 all in. Probably seems excessive but it gives peace of mind.
I previously used a high mileage diesel car and did the same but changed the oil filter more frequently - i.e, also between oil changes; then topped up the oil to replenish what's lost with the old filter. Diesel engines really don't like dirty oil so £5 (ish) for a filter and 10 minutes swapping it out, seems worth it. It was just about to clock over 200k before somebody redesigned the OSR for me - otherwise I'd still be using it.