The source of the problem you described must be something else, like clutch cable maladjustment or warped clutch plates, which can all be a source of a clutch dragging when you switch into first gear and engage the clutch. The increased slipperiness due to the friction modifiers used in synthetic oil could indeed be a possible source of clutch slippage but only under load and thus with the clutch engaged, not with the clutch disengaged as you mention.

If the bike starts moving forward when the clutch is disengaged and is in gear, that means there is excessive friction between the friction and pressure plates in the clutch. The clutch plates are not completely disengaged, and thus 'drag' each other along. Your argument doesn't hold, as the friction modifiers contained in synthetic oil reduce friction compared to mineral oils with similar viscosity. So if the bike lurches forward when kicked into gear, the 'clutch drag' problem should be even worse when using mineral oil. The 'clutch drag' problem that your bike suffered from is in fact the opposite of the problem you referred to with respect to the use of synthetic oils. The problem of clutch slippage when using synthetic oil usually occurs when the engine is working under full load, so at high revs and/or under full acceleration.

This said, forget about semi-synthetic oils, it's the biggest marketing scam in the lubrication business. Either spend more on fully synthetic oil or save your money and buy a good quality mineral oil and change oil more often. All my thumpers run on fully synthetic oil and I never had a problem with clutch slippage with any of them(*), even racing enduro's in the tropics or adverse travelling conditions (high temperatures in Africa and Asia). But that's only my experience, some might have experienced otherwise.

(*)Currently own 13 bikes. Apart from my XR650R, my four KTM EXC's (2x450, 520, 525), KLR650, BMW F650, and two XT350's all run on synthetic without any clutch slippage.