Dear ts, u prolong to 20k for oil change really messed up your engine. Whether the damage is already too much or not we do not know unless we test it.
If u dont want to spend any more money on it..u can put a bandage... Everytime pour in lucas oil stabilizer into your engine during new oil change.
Follow below steps if u want to save money + help your engine.
1. Go mechanic and do dry compression test for all your cylinders are good. If still good, proceed to step 2. If no good, u will have to do full engine overhaul because your cylinder walls already almost certain full of deep scratches and grooves. Full overhaul for damage like this includes rebore cylinder walls and putting in bigger newer pistons. Rm5k and above easily.
2. Take boroscope and look down into cylinder walls. If no deep scratchs on cylinder walls, proceed step 3
3. Buy engine flush or cleaners like marvel mystery oil, lucas complete engine treatment, etc. It is a aggresive cleaner. Follow the instructions. Not 100% but hopefully this will slowly clear the gunk around your pistons and free up all your rings and also oil passages that has been blocked. If oil consumption reduced, u can try again. If no reduce, go step 4.
4. Open up and pour marvel mystery oil directly into pistons through spark plug passage way. Wait few hours, manually turn the engine a bit, then pour again and wait another few hours. This way is very very aggresive in case step 3 does not work. See youtube to get what im saying or find a mechanic to help you.
... you mean sometimes 20k KM only change engine oil?
sometimes i do service very late but my honda city which is older and with the same practise does not has this problem