They'll just have to get out more.
Is that 3 litre engine capacity or oil quantity? My W124 takes (from memory) something like 6.5 litres, my *** even takes 8. Can't see them doing that for long.
Only one?
It's engine size but I've checked and the same price applies to all cars. It's only when you go in for an interim or full service where the engine size makes a difference.