I was always told that HM oil had additives to swell gaskets and not to break up too much crud that could be holding things together.
I was also always told that the problem is not switching from synthetic to regular oil, but from regular to synthetic in a higher mileage motor because synthetics tend to be a little runnier than regular oils and sometimes have better cleaning properties, thus it can free up crud and make any leaks you have now worse.
I ran regular oil for years. I switched my Ranger over to full synthetic after reading through some reports on it prolonging engine life and the fact that it tends to hold up really well. My F-150 supposedly had 69k miles on it when I got it. I think the first oil change or two I did in regular oil and put a shot of ATF in with it to swell the seals. Then I went to semi-synthetic for a change or two with a shot of ATF. Then I went to full synthetic (Mobil 1) and extended my oil change intervals to 10k.
My F-150 did leak oil bad. It was almost to the point that it was pouring out the side of the block through the pushrod cover. So when I rebuilt my front clip recently, I pulled the cover off to find the original cork gasket has converted to a charred crisp and wasn't sealing anything. Since putting a new rubber gasket in (Felpro) with some Ultra Black RTV to make sure it sealed good, no more leaks. Sometimes you just have to get in there and fix a leak, after seeing how bad that gasket was, there was NO oil (at least short of maybe a bit of straight crude) that was going to plug up that leak.
I had an 89 Bronco II Eddie Bauer back in college that I used as a campus beater mostly. It didn't really leak out of the valve covers and it didn't seem to be burning oil, but I was always down some. After awhile I noticed that it was spraying a mist of oil out of the exhaust. I scratched my head, asked around a bit, and finally broke down an bought a new PCV valve (I had cleaned the one that was in there twice). After putting the new PCV valve in, no more oil loss. Apparently the spring was too weak in the valve to properly control the oil flow.