Personally, I would add nothing to my oil.
Regarding the fear of leaks when switching to synthetic:
If you have leaks when you switch to synthetic they'll be more "productive".
Syntheic will not magically create leaks if your engine is leak free.
I've used synthetic for many years and in many cars and have never had a problem with leaks.
Also, while change intervals may be slightly extended with synthetic it is not wise to wait 10,000 miles to change your oil....even with a filter change and top off.
Some allege Castrol MAY be misrepresenting it's Syntec product. I beleive there is a lawsuit pending against Castrol regarding misleading claims. I'll try to get some credible info to share.
I use Mobil 1 almost exclusively.
* I'm not affiliated with either company in any way, shape or form. Merely a customer.
'85 380 SL (sold)
'85 Carrera Flatnose
'71 280 SL Signal Red/Cognac