I don’t have extensive silicone experience, but I will say this: the flowable is less likely to “stick” to the walls of the groove. This is good in that it makes for clean removal when the time comes. But it is “bad” in that I have had some pop out on rare occasions. I mean… when they pop out, you can re-apply… it’s not like they pop out after only a few days or something.
The non-flowable is a pain to manage, and can also pop out, particularly if you don’t make sure there are no air pockets in it. But it is a bit stickier in terms of adhesion to the walls of the groove. Or so I’ve experienced. All anecdotal evidence here, which is always a shaky thing. 
All that to say: if you can’t find flowable, or if you want something with colour to it (red is the fastest!) you can use RTV/high-temp gasket maker. It doesn’t self-level, but it makes a great response.