Ok so the grade discussion is 0w20 in case of 0w20 it doesnt make that much sludge as higher viscosities does. It also can vary from engine to engine some has total oil control ring failure and some has bore failure along with ring failure.
How to diffrentiate between both?
as we know its pretty hard for a normal person to diffrentiate the both type of damages but heres a hint as per my hands on experience in these engines.
1st the engines with only oil control ring failure and it should be around 30% till 60% doesnt have any type of engine knock at all and if you see the fuel trims they also are totally normal but when you start to increase the engine rpms till 4500 you see a negative fuel trim till -18 in short term ft which indicates that now engine oil has been added in the combustion chambers and o2 or AFR sensor is seeing it as a fuel and minus outs the gasoline input. In this type of engine the oil reduction can be around 500ml to 700 ml in 4000 kms plus if the vehicle is driven in local routes not more then the speed of 80 km/h with light feet not agresively 0 oil consumption os noted as the oil only is only consumed on higher rpms thus it will also has low brown sludge deposits and using a low viscosity engine oil is perfect in this situation but repalcing it earliers is the best soloution as the oil is not stressed out due to CG are mixed less hence oil is less contaminated.
2nd is the case where we see the rings detoriation along with bore failure too in this case the vehicle had developed an engine knock at lower speeds on normal load conditions like the speed of 40 to 60 km/h and also if you see the FT curves they will be already in negatives from start of the rpm from 1200 and till it reaches the 3000rpm range the fts would be around -25 so in this engine you will feel lag and underperformance along with bad fuel economy is noted.
The engine performs superb when its cols started has packed with lot of torque and power and accelerates much better till the engine is completly warmed up.
So in this type of engine sludge formation is already at top so topping up in this type of damaged engine is hazardous as you end up the oil till 5000kms like a tar gunk so 2500kms is the ideal change interval for this engine in some case i do recomend to chnage it till 2000kms till the engine not repaired or replaced. So short drain interval can help in daving your engines crucial key components. Like crank shaft bearings , Cam shaft lobes (which can be easily worn out if drain interval is extended) , less damage to already damaged cylinder bore , your oil pump would be saved also as it will not gets dirt or tar type oil to be pumped with a lot of left over gunk and metalic particles hence it will save you a lot of things in parts and repairs also the engine noise would be less.
So in a detoriated engine always change the engine oil earlier then normal to avoid more damage.