I think the oil sump holds around 6 litres . In normal city driving the dip stick wud show reduction of nearly half its level in just 100 to 200 kms travel. I always carried 5 litres of oil.
When I drove long distance I would have to top up at least a litre or two once I completed about 300 to 400 kms. The oil light wud come on. The funny thing is I wud never experience drop in power or performance and no smoking from the exhaust.
The first diagnosis was a bad turbo. We checked and the turbo was spraying oil. This was changed. The oil consumption did not stop. That's when we opened the head to check because suddenly i started experiencing a drop in compression in one of the cylinders and then a bit of engine missing started. Now I have replaced the valves, the stem seals, the set of hydraulic tappets,