You can do either.  If you go with Synthetic you can extend the oil
change intervals.  Mobile One now unequivocally says that in "typical
use" you can go 15,000 miles on their newest Synthetic.

As another poster points out, you can extend the oil change intervals
by switching to synthetic.

For the "is it worth changing?" discussion, my analysis suggests that
you need to expect to drive the car less than about 125k on synthetic
oil, and then avoid an oil-related failure during that time, a failure
that would have happened if you had been using conventional oil.
Since the car has probably not enjoyed a steady diet of synthetic
during its first 100k, and has a statistical life expectancy of
considerably less than 225k, the recommendation would have to be
regular changes with conventional oil for its remaining life.

Synthetic may provide "extra" protection to oil-wetted parts, but that
may not buy you anything at this point in the truck's life.  Other non
oil related things will fail due to age or neglect.  At some point
that list of things to repair will reach a level where a newer car
will be a good decision.  So there you will be, with the bottom of the
engine looking brand new, while the rest of the car is deteriorating a
lot faster.

M7 '92 has  almost 200 k on it now, Mobil-1 since new.  I had the
rocker covers off at 125k to replace leaking gaskets, at which time
the moving parts looked like new.  Heads off for headgaskets soon
after that at 140k, no ridge and I can still see tthe original hone
marks.  OK!  By 200k, the engine probably needs a timing chain, and
maybe a valve job soon, and the original (!) auto transmission is
starting to show signs of wear.  The headliner has fallen in the back
and needs to be replaced, driver's seat is worn to the point of
needing new leather, but everything else seems fine.  Is the car worth
saving?  Probably, thanks to sythetic everything since day one. I
drive the Porsche most of the time when I'm home, rental cars get the
bulk of my miles overall.    Will your car last long enough in your
care to reach this state?  Most will rust, break, or get wrecked or
damaged beyond cost-effective repair long before they reach 200k.
Want to spend two to three times as much on synthetic oil changes to
reach this used-up car nirvanna?

Cheers!

dr bob