Two oily kittens-advice please?

Hi, I am the neurotic mum of two beautiful kittens Sencha and Chai (seen as my avatar) from Sydney. I've recently noticed that my two playful kittens have developed very oily coats over the past few days. Both are otherwise healthy and grooming themselves as per usual. I have changed their dry food over the past week from science direct to something a bit more affordable (whiskas here in Australia)

Any suggestions as to (1) the cause and (2) what I should do?

Thanks!

did you change the food before or after the oily problem started?