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Oil changes

i have three cars. a 2000 ford Taurus with 97,500 miles, a 2000 town & country with 92,000 miles and a 1994 dodge grand caravan with 145,000 miles. I change the oil on all cars between 2,500- 3,000 miles w/conventional oil. i would like to switch to synthetic oil, would i have any problems with the engines if i switched? oil leaks, etc.

Changing to synthetic won’t cause any problems, but what do you expect it to do for you?

i would like to extend the time for oil changes, to 5,000 - 7,500 miles.

Unless you do a lot of hard driving, or in extreme conditions, (dusty,too hot, or too cold), I would go 4-5,000 miles between changes anyway. Conventional oil will hold up to it.

You can do that, but changing to synthetic really won’t be a big factor there. What allows for longer change intervals on some cars is that those engines have less blow by and so don’t contaminate the oil as badly. Synthetic or regular oil both suffer from contamination equally.
Your cars are older now, if anything you might think about shortening the interval now instead of making it longer. Of course this is speculation. If you want some certainty about it, you should have your oil analyzed to see how long you can go between changes.

Oil changes are like religion. Everyone has a different faith. Here’s mine.

Check your owner’s manuals on the recommended oil change intervals for those cars, paying close attention to the definition of “rough service.” If you drive under rough service conditions (like mostly very short trips, or towing) then stick to the recommended interval, and stick to cheap conventional oil. Besides lubrication, oil also acts to carry contaminants away from the engine, and to make that work you have to change it.

If you drive “normally” according to the manual, you can extend your oil changes to that interval using conventional oil. Except that if the recommended interval is more than 5000, I personally would change at 5,000 using conventional oil, and only go the full interval with synthetic or a synthetic blend.

Always change at least once every 6 months no matter what the mileage is.