Lol    I get my engine oil very cheap.. And my car is at 200k mileage now.
I bulk buy a dozen Bosch Oil filters for RM68.50 from lazada last time. On top of that I get my engine oil from paulhew in Shoppee. He sells Castrol Edge quite cheap, with coupons and free shipping it only cost RM110+ a bottle.

I only drive like 16-20k km a year on average now and per day driving time will be around 2-2.5 hours due to traffic jam. I noticed that my engine torque and acceleration affected a lot after I started getting stucked in traffic jam. Previously i was working in Cyberjaya and to travel from Bukit Jalil to Cyber is by highway and rarely have jam. Hence this is why i started changing engine oil before due time (you are supposed to change engine oil every 6 mths anyway) and my engine power sort of restored a lot.

To be honest there are a lot more to engine oil than just the dirt level from an engine oil blot. Engine oil contains friction modifier, detergent, viscosity modifier etc etc which wears out over time from heat and usage. A clear engine oil doesnt show the amount of wear and tear to your engine internals. I used to extend my engine oil service life quite frequently last time when I was a student. my Semi Syn oil would last for 12k km, Fully Syn stretch to 15k km. But the darkness of the oil at each service change scares me.   

My rule of thumb is, if u feel a major difference in performance after every engine oil service change, you should bring forward your oil change. The extra drag in your engine waste fuel anyways and will just offset the savings you had extending your oil change duration.