Overhaul...at 88,000 kms

[4JGtootsie]

This is a misconception on most drivers. To help clear the intake and exhaust ports of deposits or sludge, I would run the engine hard from time to time. This will ensure also that the engine is at the top of its' performance.

This is correct. Run it hard na parang may LBM ka, it will flush out carbon deposits sa valves and seats, clear sludges sa fuel lines and cough out the soot build up sa muffler. Do it time to time because carbon deposit will solidify like cement, parang brownies na naiwanan ng matagal sa oven.
Lagay ka na rin the hose with water sa tambucho para malusaw ng kaunti ang soot before you run it hard.

[4JGtootsie]

"the kamuning mechanic is not at fault here guys. it is the grime in the valve stem that caused a valve not to seat properly, this problem usually are caused by oil additives, or certain brands of oil."

This is possible. But he said his car was working fine before he brought it for a routine valve adjustment. An unseated valve will cause low compression leading to poor burning of fuel thus grime in the stem and on the valve seats.
Good compression will give an excellent fuel burn provided all valves are sealed during compression and there is spark sa spark plug.