I've always used 10W40 never have had problem no leaks. But I have been thinking of 10W30 next time.

 The synth oil is great stuff reguardless of weight.It does have smaller molecules[and they are all the same size,in dino oil they are larger and random in size],but it leaks in older vehicles because the seals and gaskets are old and have shrunk somewhat. The dino oil with larger molecules won't go past the seals,but the synth will.
So it's not the oil that causes the leaks,the oil leaks due to the aged seals and gaskets.
It also is not recommended to break in a new engine with synth oil as it is to fine a grade and will not allow the bearings,seals,and rings to set correctly.

I run 10-40 Castrol for about 100k now, no problems what so ever.  

Synthetic (I use it, Castrol, in my car and other truck) works great, motor is clean as a new one, but it is pricy and if you drive a lot it will cost you. It doesn't cause leaks, it just exploits them if there are small holes that dino oil won't make it through.

<SNIP>
Have you tried Valvoline MaxLife?  I love the stuff.

Just make sure YOU change your own oil, and don't let Valvoline Instant Car Destoryers Oil Change geniuses Employees...   :-/