I can only relate my personal experience in using synthetic oil. So, take it for what's it worth.
I had a 1990 Chevy Ext. Cab P/U truck (gas) with the short block 350 engine and a 5-speed manual transmission. I sold it 11 years later. When I sold it, I had 377,000+ miles on it but still had the orginal engine and transmission. Up until 100K miles or so, I has used Pennzoil 30 wt. After that, I started using Castrol Syntec 10W-30 for the engine and Royal Purple Synthetic in the transmission. Did the synthetics really account for the high mileage? I don't have a clue but I did not have any leaks, until just before I sold it. One time, when I was driving to SE LA from Amarillo, TX, (about a 12 hour drive) in August, I noticed that when I was in bumper-to-bumper traffic in Dallas, the temperature gauge would get uncomfortably close to the red line. However, when I started travelling at regular I-10 speeds, the temperature would again drop to just about normal levels. When I finally got home, I checked the radiator level and was shocked to find that, if there was water in the radiator, I couldn't see it. Come to find out that a ceramic fitting at the top of the engine manifold was cracked and the coolant was vaporizing away, while I was travelling that day. Now, I doubt that the radiator was completely empty but I do know that Castrol Syntec 10W-30 was in my engine at that time. Would a regular oil have endured this stress? Once again, I really don't know but I'm glad I didn't get to find out.
My wife drove a Camry for 240K+ miles before she got a new vehicle. I had used the Castrol Syntec 5W-30 after the Camry had about 1K miles on it. No engine trouble at all. I do my own oil changes. One time I was out of town on an extended project and did not have a chance to change the Camry oil (and I forgot to mention to her to have it changed). When I got back, I finally changed it and realized that it had been just over 18K miles since I last changed it. I certainly do not recommend this practice but it had no ill effects (at least this 1 time).
Finally, I have a 2001 Tundra with 115K+ miles on it. After the first 1K miles, I began putting Mobil 1 10W-30 in it (sometimes 5W-30,when Wally World is out of the 10W-30). So far, so good.
One thing I do like about using synthetics is that after each 5K miles, all I do is change the oil filter and top off the engine with synthetic oil. Then, after the next 5K miles (or 10K miles total), I'll do a complete oil and filter change. This helps lessen the cost of synthetic vs regular motor oil. Another thing I like about synthetic oil is that I do not have to change the oil precisely at the 5K mile mark. With regular oil, I'd not go beyond 3K miles. This may not be necessary but old habits die hard.
Just my $.02 worth. YMMV.