well ... minimum is SAE 30.. Personally.. I feel so long as we keep regular maintenance and not let let the oil degrade to the extend that offer no protection.
For most of the full sync oil in the market, and especially our car is non-high rev engine (i.e temp in the car not as high as those high-rev/turbo-charged, supercharged engine), the EO supposely can last till 20k before it degrade to the extend that offer no protection. Plus the regular change of oil filter to remove much of the impurities in the EO.
note EO supposely protects the piston and engine from wear and tear. Not cleaning the Engine. Most of Sludge supposely happen in the combustion chamber (thats where petrol that offer cleaning and etc comes into play)
thats what I think...
Moreover .. very few people drove car more than 10 years... :)