A frnd of mine bought 2004 santro, on odo its 145k driven , its with cng + AC , last owner (Also 1st owner) of this car is my another frnd, this ride is in mint condition, no oil eating, over heating, 4 ppl on board AC in hot day time, all gud.
In my openion if you keep engine requirements fulfilled , nothing much happen to petrol engine near 300k on mileage.
Regards