Adding butterscotch syrup in primary

It may be worth noting that a form of "butterscotch" flavour - often found as diacetyl - is a by product of yeast metabolism and can be accentuated by the fermentation process. If diacetyl is really required why not adjust the brewing method and let the yeast do the work. A typical beer that is produced with elevated diacetyl levels in Sam Smiths Tadcaster Special. Generally speaking if one is to add a synthetic (or natural) flavour it is often best added after fermentation is complete or nearly so. The point being that endo and exo yeast metabolism can alter flavours noticeably (eg bio-transformation of hops).

I have never used that for brewing but i have for ice cream.

I would add it after the the Lag phase of fermentation, basically in the heart of fermentation, when the yeast is done multiplying, due to the Potassium Sorbate.

I would start this in a small batch, probably gallon or less, or a fast ferment to see how much butterscotch flavor is needed. Then scale it up from there.

personally i would make it myself and take a hit to head retention. butter brown sugar and heavy cream, vanilla is all you need.