I used to live in Karachi and drove to areas many people on PW have not been to in Karachi. my car - a diesel VW Golf lubed with caltex delo 400 (aka silver) would be perfect with 5000 kms interval - if I used the highway a lot it was extended to 6000 or sometimes 7000 kms. Engine was perfectly fine with that. It had a 4 litre sump and oil filter that holds 1 litre in itself.
Petrol engines were also treated the same with regular off the run oils like caltex havoline formula or similar, even in fleet operation that oil was perfectly acceptable to use till 5000 kms in urban driving. The little factor of temperature is critical for long life, its when the oil additives work and also steam is vaporised out of the oil and sucked by the PCV.
If you want to see a clean engine - I will take a picture of my engine next time I open it for something. Its oil is changed at 25,000 kms with synthetic lube. I do run it hard now and then and use a fleece type long life oil filter. Its real clean. It even has no varnish deposit - thats how clean it is. It has been driven nearly 169,000 miles (270400 kms)