Only use grease in rolling element bearings – an appropriate grade of oil for everything else.
Using grease on the slideways is a guaranteed way of reducing the life of your machine – the grease holds fine swarf, mill scale, rust etc. & forms a very effective grinding paste. Regularly applied oil flushes these normal products of using the machine out of the slideways.
Look to industrial practice – CNC machines with conventional slideways use automatic systems to regularly apply slideway oil directly to the moving elements. Grease is only used in machines with linear bearing guides – even then, most machines so equipped tend to use oil in my experience.
Regards,
Nigel B.