Is an engine flush necessary?
A good engine flush can help loosen deposits and dissolve sludge, returning your engine to like-new condition.
However, in old engines with high miles, sludge may be the only barrier keeping oil from seeping through worn or cracked seals.
Removing the sludge exposes the seals for what they really are – junk. Soon, your engine begins leaking oil, and your mind instantly associates the engine flush product with an oil leak.
In reality, the seals were already bad; the flush simply revealed their true condition.
If you suspect your vehicle falls into this camp, leave well enough alone and skip the engine flush.
It’s probably not worth trying to revive an engine in such poor condition without first fixing the bad seals or other defects.
In effect, you’re choosing your problem: either sludge and deposits robbing performance or if you clean the engine, the seals showing their true condition.
While not a required step when
switching to AMSOIL synthetic motor oil, we do recommend flushing your engine if you want to give your vehicle a fresh start. (I think my belief was like this, when we switch brands, or even different viscosity grades, a flush will be a better job).
Source:
Engine Flush: Is It Good or Bad for Your Engine? (Plus See How To Do It).
Maybe I might have missed reading,
@allhyundaicars - "Sir, had you asked them whether you can source the oil and whether they will do the job?" when they told that they don't have 5W30.