From what I have managed to piece together about leaks with synthetics (from lots of reading) is this:
Changing to synthetics will not in itself cause leaks, synthetics have a higher detergent level and will eventually clean your engine. If you have leaks that are currently being plugged with oil sludge, the synth will clean out the sludge and you will get a leak. But, synthetics are also better for seals, apparently they have additives that will swell seals, so with a good quality synth, your leaks should eventually go away.
I purchased my 97 PGT at 191,000 km, I don't know what oil it had in it (but looking into the oil filler the cam gears were spotless, so maybe synth), but I switched to Fuchs semi-synth first oil change, I used that for about 20,000 km, I have just started using Nulon full synth. I have minimal oil loss, 1L max between changes.
I did initially have oil coming into the plug sockets, but I cleaned this out 15,000 km ago, haven't seen any in there since.