Hmmmm... I'm not really sure about this coz i change my engine oil every 10,000km or 4~5 months whichever comes 1st... My bro n me been doing this for quite sometime now... My bro car has already reach 120,000km in the span of 3 years n still haven't need to do any overhauling ler from the sludge thingy or whutever... My 1st car reach 100,000km milage too b4 i sold it n it never needed to overhaul b4... My current car is now at 40,000km n still ticking..

I think maybe it depends on the oil quality n how frequent u change type of lubricants during ur car lifetime... be mindful also that everytime u change ur engine oils not all of the dirt and grimes would come out with the dirty engine oils.. Some may still be in ur engine so it is not suprising to find sludge etc in ur engine..

My job involve with dealings in the petroleum sector n i had talked with a few engineers from oil company's like caltex n petronas n they all advise against the habit of frequent changing of oil brand/grade/type n such.. They say if u really wanted to change u should atleast use the same brand/type/grade for a while (maybe after 3 service periods) only after then u can try other brands/type/grade. No engine oils are made alike in terms of formula n such.. n u can't really see/feel any diff with just one time trial service using a specific brand/type/grade... Sometimes it takes a longer period to see the difference.. Maybe after 2-3 service period..