Oily baits are less effective in colder water temps regarding the release of attractiveness from the bait but from my experience barbel certainly don’t find them any less tasty at a particular time of year.
There is an awful lot of nonsense on the bank spouted about baits for barbel and in my honest opinion the worst thing you can do is carry loads of different types lacking real confidence in any one in particular. I actually don’t use pellets at all. Not because they aren’t any good as we all know they are brilliant but because my confidence is at 100% With another bait and there is no need to change.
Pick a classic, use it, catch on it, gain confidence in it and stick with it. I make my own boilies now and have absolute confidence in them because barbel absolutely love em