Don't tell him that, he'll bring it to the "guy down the road who changed
his own brakes once".  ;)   For that kind of an issue, you need a real
mechanic with his own shop.  If you see a hoist for swapping engines out,
that's the kind of guy you want.  Better yet, a Nissan dealer, if there is
one around you trust.  They're really hit or miss.

I brought it to Pepboys for an engine diagnosis already. And they found
nothing's leaking. BTW, I got a response from another forum. The
replier's an experienced mechanics (well, he claimed).

"Check for sludge buildup in the engine. Remove the oil fill cap and
look for a jelly-like substance in the engine. If you see that, you
will have to remove the valve cover and check the oil return lines in
the head. They may be clogged, causing the oil to fill up in the head
and enter the combustion chamber through the valve guide seals."

It sounds reasonable, doesn't it? But I don't think I am capable enough
to check it myself without any tools and stuff. I will probably bring
it to the shop sooner or later.

Thanks everyone.

Leo