Thermal fan clutch

Yes, that is how they work. They should not be slipping more. However, the fluid inside can be thicker before start up. IF you suspect an issue with the fan, I would suggest a MP part called the viscous fan. There a funky off set bladed fan, 5 blades and were found on HP engines, not regular engines, like a 318. They will move a ton of air. You also have a fan shroud correct? If not, grab one.