With the miles on the engine I would consider going to a high milage oil. I used the valvoline maxlife (I believe that was the correct name) 10w30 and my usage did go down a bit..if that's an issue for you. My old motor carried 200,000 miles on it's back and seemed to prefer the high milage stuff. The real key is keeping the oil clean and using the best filter available to you..in my case I chose the K&N filter, it's my motor and don't mind the extra 10$ every change for peace of mind. Synthetics CAN but not WILL, because of the detergents in the mix remove some of the oil residue and build up, creating leaks where non were previously found. Every engine is different..anyone who says it will or won't is incorrect..the only true way to know is to find out for that particular motor. Synthetic oil is an excellent choice, but my last motor survived 200,000 on dino oil...so I did not see the point of switching as there was no gain I could see for me. My current engine still being quite virgin sees castrol 10w30 and will eventually switch when more miles are added to it.
I love synthetics, but with that many miles on it, I would say keep the dino oil, or go to some high mileage miles. I put synthetic in a car with 140,000 miles, and no leaks. But I knew that that oil had been changed regularly, which I think helps with the buildup of sludge. Like DMAN said, every motor is different.
it still is recommended, its just that the majority of people would not want to deal with an oil leak. If you are not scared of an oil leak, then by all means go for it. synthetic is better in every way compared to dino oil. But dealing with an oil leak is something that you might have to do, especially on an higher mileage motor where there might be some sludge covering up some leaks. like DMAN said, the detergents in synthetic might remove some of this sludge and reveal a leak.
I switched to synthetic at about 89K, and it did start leaking out the rear main. I usually use regular or semi-synthetic in the mustang since I change the oil very often, so synthetic would be a lot more $. We have three other newer Fords that use the thin oil, and all I use in there is the Ford semi-synthetic brand you can buy at Wlamart for about $2.25 a quart which is pretty reasonable.
Full synthetic is defintely better, but not worth the $ if you change your oil often like it should be IMO.