When acrylic colours first became widely available in the 1950s and 60s, there was a reluctance to recommend overpainting them with oil colours. Because acrylic colour does not become brittle with age like oil colour, it was feared an inflexible oil colour film over a flexible acrylic film could lead to cracking.
In practice these concerns have not been confirmed, and at Winsor & Newton we are not aware of a single case of cracking caused by painting in oil colour over acrylic, either as a complaint from a customer or in laboratory tests.
But if you are overpainting acrylics with oil, you should keep the acrylic layer thin, absorbent and fairly “lean” to limit its flexibility. It is still not recommended to paint with oil colour over thick impasto layers of acrylics, because of the differing levels of flexibility between the layers.