you have basically three options:

1; leave it as it is and put up with it till it's warm

2; try a thinner grade oil during the winter months

3; replace all the followers.


changing the oil to try it is fairly painless, just costs you a few ££ and no bad can come of it.

Replacing the followers may help but there's no guarantees, especially if it's actually the oil thickness which is causing the problem anyway. If it's OK when warm then the followers themselves are unlikely to be the root cause. There is no tappet adjustment, they are automatically "adjusted" by the oil pressure.