I have used 100% synthetic since 1977.
It has without question been FAR better in every way than any petroleum oil.
On 2 vehicles a 1999 Lexus and a 1990 Honda Prelude both got better gas mileage, the Lexus I was able to use a lower viscosity 0W-30 instead of 5W30 that lower viscosity plus being synthetic boosted my MPG from 26-27mpg up to 30-31mpg on straight highway miles, so roughly 3mpg.
Also being one of the worst offenders to "sludge" the engine has instead remained sparking clean with ZERO sludge.
This engine WILL sludge using petroleum oil and I dont care if you change it every 1,500 miles because how it sludges is when you get the engine hot and then turn it off the top of this engine gets red hot and as the oil runs down after the engine is turned off a thin film is left to BAKE on this red hot top end.
That thin film turns to sludge layer after layer IF you use petroleum PERIOD.
Changing to synthetic which has a MUCH higher flash point stops that problem plus the detergents in the synthetic oil clean the engine nicely.
I would not even own this car without synthetic oil.
The Honda I got about 1-2mpg better mileage.
All other vehicles I 'may" be getting about 1mpg better but hard to say.
The reason I started using synthetic was when I used to have a nicely built 2180CC VW Baja Bug, quick as hell but would lose power once it warmed up.
The throttle in cold weather and right when I started it was like a motorcycle, had instant petal response and I could actually feel it lose horsepower the hotter it got. I tried everything, added a deep oil pan and sump pump, added a large oil cooler and then put the cooler on the roof under a scoop all of which added several quarts of extra oil which helped a bit but just took a bit longer to lose power.
One day I tried synthetic and BAM, problem solved, NEVER lost power again even in summer while running all day in the desert.
So nobody can tell me that synthetic does not make any difference, I KNOW for an absolute fact otherwise.
It also does NOT cost anymore in the long run as you can easily get 7000-10,000 miles out it instead of 3000, so what have you saved by using cheaper petroleum oil and changing it 2-3X as much? ZIP
As for the OP question, using synthetic is no problem at all in any engine at any age as long as that engine has not over heated and baked its gaskets or sludged and dried out its gaskets, in this case it could possibly leak after the synthetic cleans the sludge away. Although it is also just as likely that it could swell the gaskets back up and reseal itself depending on how bad they are, but either way ANY leaks were there to start with.
I bought my 2002 Sport Edition with 158xxx on it and immediately changed to synthetic without a single problem, I end up spending a bit more each year using synthetic instead of petroleum solely because I change the oil in a year regardless of mileage and I only drive about 5000 miles a year usually, BUT I still use synthetic and do not mind spending a few extra bucks to guarantee I run the best oil, keep the inside of the engine clean.
If you average more than 10,000 miles a year then synthetic will not cost you any more than petroleum, if you travel A LOT of highways miles per year then you could actually save money running synthetic.
So there is absolutely zero reason to not run synthetic.
I run synthetic everything, ATF, Gear lube, everything.
If anything petroleum oil is on its way out to be replaced completely by synthetics. In fact lubricants are probably on their way out within the next 50 years as most likely everything will have to go electric before much longer.