Midnight Rain, it's not age gap but both are dilfs.
And you know what's better than one dilf? Two dilfs
They said straight romance (￣∇￣")
My reading skills are not the best
https://www.mangago.zone/read-manga/uncle_cool/ (she's 16 though but I don't remember anything happening between them, I might be wrong)
https://www.mangago.zone/read-manga/doppio_senso/ (not THAT big of an age gap but he was quite older from what I remember)
https://www.mangago.zone/read-manga/reemployment_life_with_an_unsociable_untalkative_magician/ (gilf, for grandpa, more than dilf lmao. It's very good though)
https://www.mangago.zone/read-manga/another_typical_fantasy_romance/ (he's not older but LOOKS older and is atipically rough-looking for a male lead, I love him)
https://www.mangago.zone/read-manga/saikyou_no_kurokishi_sentou_maid_ni_tenshoku_shimashita/ (not really that focused on their romance, even in the novel)
https://www.mangago.zone/read-manga/the_flowery_path_of_evil/ (he's not a dilf... at all... but he IS her father's age lmao)
And then there is the classic Violet Evergarden (novel only), even though THAT relationship might be quite controversial because they met when she was 14... well, let's just forget about that, I see nothing, the story is amazing :D
this please he is so hot https://www.mangago.zone/read-manga/sesame_salt_and_pudding/
Age gap straight romance where the guy is a lot older please i love dilfs give me dilfs