It's more the other direction. Most of the oil being sold as "synthetic" is really just highly refined petroleum pumped from the ground and that includes the lower grades of Mobil-1. Good luck getting the makers of Mobil-1 to admit that. Some of the higher priced stuff like Royal Purple, Amsoil, etc are really synthetics. Oils like Pennzoil Platinum are synthesized from methane so you could call it synthetic. Blah-blah-blah...Isn't most "regular" oil these days a synthetic blend anyway?
That's a loaded question. You can't just say "synthetic" and get a clear answer. On the other hand...Looking for advise on switching from regular oil to synthetic. Pros and cons
There is a large difference between a conventional oil with high mileage additives and a first tier high mileage synthetic. Sludge isn't from the conditioner additives, but from the base breakdown, top end blow-by, and recirculation of exhaust gases. Seal conditioners primarily protect the seals that are there to prevent fluid loss through leaking out of the engine..I would stay away from any oils specifically designed for high mileage cars. They have seal and gasket conditioner additives in them that cause engines to build up sludge over time. I use Mobile1 extended performance 20,000 mile oil myself. However, I change my oil and filter every 5,000 to 6,000 miles. The added detergent in the extended mile oils remove oil deposits more effectively within the engine keeping it clean and clear of any buildup, but that only works if you change your oil and filter at or around the intervals I change mine at. 20,000 mile oil is a big farce. It's the oil industries sketchy way of marketing misguided convenience to the consumer. Oil is cheap, engines are not.