Brother, in cars where its suggested to change engine oils after 10-15,000 Kms, or even more; they have large engine oil sumps, and special arrangements (If engine oil level drops, then simple top up is reqd.). Their heat dissipation, surface contact area, etc. Is more IMO. And I am talking about 6-8,9L engines.
This analogy cannot work in 1300-1500 cc small engines with small oil sumps, i.e. oil sumps are around 3.7-4.2L. In small engines, heat generation will be more and dissipation may be comparatively less, compared to large engines, and if you'll keep on dragging that oil for 10-15,000 Kms, or even more, you may ultimately end up in damaging and destroying your engine, as engine oil may fail to perform its intended function.