MTRAN2000 has a point. do it first.
this is what i will do, it depends how much deposits that the inside of the engine has.
Then if you have extra money you can oil flush it for about 10mins with Toyota Engine Oil Flush chemical OR just oil change it.
Then clean that Engine Oil Cap.
If that is mine, I would drive the vehicle until it reaches 500kms, then oil change it again but do not add an engine oil flusher.
then monitor it until the next engine oil service (i would do it every 5000km) if it still showing sludges under the engine oil cap, well there's too much to remove still
unless you want to open the top of the head up and/or engine oil pan.
questions aside:
1. was the vehicle coolant reservoir level low or getting low or noticeable low?
2. where do you bring your car for oil change? do you follow the 8k oil change service?
3. if you do it yourself, do you put the right engine oil type?
5. do you have the engine oil cap photo taken? can you show us?

4. is there a sign of too much burning oil? check your oil level every 500km when your monitoring it
