Found this on a Mechanics website:
Using synthetic oils in older models
When using the term “older” to refer to cars, it means those manufactured before 1990 or so. The risk with these models is that the seals and other components often aren’t as tight as with newer models. The synthetic oil could leak out and the vehicle could burn or lose oil. What this means for you is that you would be replacing your oil and having to monitor the levels more often. If you didn’t, you would risk damaging the engine or other components. Because synthetic oil does a better job of cleaning out sludge, it could remove deposits that are acting as seals.
It’s not accurate to say that you should never use a synthetic oil in an older car. If the car has been maintained and is in excellent running condition, the synthetic oil may protect the vehicle and prolong its life.
Signs of problems with synthetic oil in older cars
If you decide to switch to synthetic oil for your older car, talk to a professional mechanic first. He may want to check over your vehicle before advising you. If you make the switch to synthetic oil on your own, you will notice oil leaking out from the vehicle if there is a problem. You will need to have a mechanic look at it and make any necessary repairs or replacements. It's important to read the owner’s manual or talk to a mechanic about any changes you want to make with the type of oil you are using to protect your older model vehicle and ensure its long life and continued performance.
