What makes you think its the fans? Is there a sound from them? You may try to replace thermal paste. If the fans are failing, replace them with 2 usual 80mm fans.
No they sound normal, just the cooling degradation. Does the paste fail?
Yes, the paste can fail - it dries up. You can see its bad when you separate the heatsink from the processor.
You can pick up a new
7970 fan on ebay, which may be worth changing while you have the cards off line. But start with a good vacuum clean through the heatsink vents.
The oil you see probably isn't from the fan, but from the heatsink pads for the ram chips.