Synthetic oil has very powerful detergent, up to 10 times more powerful than
conventional oil. If the engine is dirty
inside, a flush should be used first. Another approach is to add 1 quart of
synthetic to your next oil change, and watch the stick for gritty buildup.
Older cars get neglected, old engines dirty oil faster than newer ones, and
contaminate oil quickly.with dirt and blow by.
If the engine leaks conventional oil, it will leak * more *
synthetic, as synthetic seems to find leaks.
A healthy engine will run cooler, last longer, and deliver more power
with synthetic. Also consider synthetic transmission fluid, same detergency
issue applies, a few changes may be needed to flush dirt.
I've been using synthetic oil, and trans fluid for over 10 years, and
would never consider using anything else.