Castrol 5w50 full synthetic oil....

Certainly don't want to digress into a "best oil" discussion. But in my experience, I find full synthetics to leak a bit more in our older engines with their old-school tolerances and out-dated sealing. 

When synthetics started to become popular we were supplying engine bearings to many race teams.  This goes back to the late 1990’s, we started seeing engine bearing failures in those race engines.  The common issue involved was the race team had recently switched to synthetic oils.   We know all the advantages a synthetic can offer but one thing we learned the hard way back then was synthetics will flow more oil through tight places than the Dino oil it replaced.  These old school race teams ran .003 or more crank and rod clearances back then.  Worked fine with regular 20-50W race oil.   When they switched to synthetics we saw some bearing failures.   The solution was to tighten the crank and rod clearances  to .0025 max.  Teams did this and the problems went away.  The synthetic oil flowed better which in most cases was an advantage but with loose clearances it lost the ability to protect the crankshaft and rods in engines worked at extreme levels.   Since then I try to hold .002 clearances on rods and mains in engines I build.   I don’t race but this gives great oil pressure and you can run any oil you wish.  All of the OEM engine builders tightened the allowed oil clearances also.,  Toyota adopted running crank clearances of .0015 back then and look how long those engines last but Toyota also took great lengths to make sure the main girdle was straight and perfectly true and check every bearing very close to make sure there is zero variance.   Aside from long life they said this improves Noise, Vibration and Harshness, NVH in these engines.  Long story to share Synthetics flow better and will flow through smaller holes than regular oil will flow through.   A well sealed engine will not leak with either oil but not all engines are well sealed.

No oil (conventional or otherwise) is going to break down during off season storage- that’s not a reason to consider switching to synthetic IMO. Now, if you like the better heat handling properties of synth, that’s a consideration.

The 5w is on the light side and not sure id use it, personally. Mobil1 (15w50) and VR1 both offer synthetics with weight ranges that are closer to what you’re using now. There’s also a mobil Delvac 15w40 that looks pretty good (and is supposed to have high zinc/phos).

Regarding leaks, most of the less exotic synthetics are using base stocks that are fully compatible with conventionals, so they’re not going to ruin seals or cause leaks, contrary to some of the old myths still floating around on the web. Lighter weights can certainly exacerbate existing leaks, though.