Mixing oil and grease in a gearbox

Not sure if this is the correct place for this but the Petroleum engineer section seemed to be for exploration so here goes.
We have a gear box that we have been using for a while now. We had been using oil in it but the manufacturer suggested using grease. We switched to the grease he suggested and started seeing bearing failure. We are planning to go back to oil but are wondering if there is a problem leaving the grease in there with the oil, or if the grease needs to be cleaned out. The thought is that there should be no problem with them both in there together but I have not been able to find anything to confirm or deny that idea.
Can someone point me in the right direction?

Thank you