What I’ve heard about synthetic oils in older engines is that synthetic tends to find leaks that dinosaur juice won’t get through.
I’ve heard that the synthetic will dissolve the gunk that keeps everything sealed in an older engine.
1. If you have a new car and plan on keeping it for about 125,000 miles, you are wasting money on synthetic oil because of the cost of synthetic oil.
1. If you plan on keeping it "until the wheels fall off," synthetic oil will ensure that the engine runs indefinitely (my 2000 Malibu has 225,000 miles and does not burn any appreciable amount of oil and I change the oil and filter every 6,000 miles and do not need to add oil between oil changes).
3. If you buy a used vehicle with less than 50,000 and is less than 5 years old, you can change to synthetic with no issues (but check the oil every 500 miles just to see if you are burning any oil).
4. If you have a car that is older than 5 years and has more than 50,000 miles, you will probably start to burn/leak oil, because the engine has begun to wear and synthetic oil will find those wear points.
5. If you convert to synthetic oil from regular oil and you start to burn oil, you are still getting the protection from synthetic oil, you just need to add oil between oil changes. For instance, in 1992, I bought a 1987 chevy caprice (307 olds engine) with 87,000 miles. It was burning about 1/2 a quart of oil per 1,000 miles. When I switched to synthetic, it started burning about a quart per 1,000 miles. I got rid of the car in 2005 and it had 260,000 miles and still only burned a quart per 1,000 miles. I got rid of it, because it was rusting from the inside out; which cannot be corrected. When it had about 210,000 miles I had to replace the intake manifold gasket; which allowed me to look inside parts of the engine. The inside of the engine was spotless- it looked it had been steam cleaned and coated with oil.
6. If you convert and older car to synthetic there is a risk that when the synthetic will cause a piece of sludge to clog up an oil return hole; which can cause engine failure. My 55 year old mechanic has only seen this happen once in his 30 year career.