Please note: The minhag in EY (for both Ashkenazim and Sephardim) and for Sephardim everywhere is to say מוריד הטל whenever we don’t say משיב הרוח.
This is not generally the minhag for Ashkenazim in chu”l. But I think it’s possible that they may be allowed to if they want to, so imho, it’s kidai to ask a sheilah to find out if you are allowed to, for the above reasons, especially if you find that you often forget.