My reason for using semi, not synth, is that the synth is known to cause clutch slip in some bikes, with some oils, and the semi isn't.
If you know your bike has no problems running on any synth, or on a particular synth which you can always get you hands on, then there's no reason not to use it.
If you don't know that and you're in a place with a limited choice, then a semi is the safe choice.
If the only bike shop in town only has synth in stock, then use it. It will probably be fine, but if it does cause clutch slip that is nothing you can't live with for a few days until you find another oil.
Semi does degrade quicker, but so long as you change the oil at the correct intervals, this shouldn't be an issue.
The point of this post is to assist the OP in making a decision, the OP is never going to get universal agreement and a firm answer from a forum, but if people post their reasoning, at least then the OP can read it and decide for themselves what they agree with.
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