Yeah, break in on conventional motor oil. You need the wear to seat rings, etc. Synthetic base stocks are more stable and do not degrade nearly as quickley at higher temps. They also have less change in viscosity as the temperature changes (higher viscosity index, vi). Some newer motor oil base stocks are hydrotreated (saturated with hydrogen) to increase the vi which gives a more stable viscosity.
I've run Mobil 1 in most all my engines (after break in) and have not seen increase in consumption or leakage. Syntec has a hydrotreated base stock. I use Castrol conventional motor oil, they use a very good additive package in their oil.
Remember, auto manufactures want you to run the lower visc oils (5w 's) to get better fleet mile per gallon numbers (not intrested in engine life). I've never run below a 10w- oil and had no problems.
PS I keep most vehicles atleast 10 years and usually well ovewr 150000 miles