100% fan speed is not good as you will wear out your fans in no time. Best to try and keep fan speed below 70% for longer lifespan of the fans. Now you also need to keep the temperatures cool, your 60C is a good goal to aim for. You may need to separate your GPUs and either relocate them to an cooler environment or direct an external airflow over them such as a box fan. The other thing to watch for is power surges or brownouts. A good surge suppressor in front of your PSU is recommend.
As far as lifespan of the cards themselves, I have some 3+ year old cards still mining away. Their mining efficiency decreases over the years simply because newer more efficient cards are released, but they are still working just as good as the day I bought them.
But fans can easy be replaced and cheap enough. Fans are consumables. Besides fans with ball bearing have lifetime about ten and more years.
Not always - my Sapphire HD7750 had a dual ball bearing fan, but the fan lasted less than 5 years.
On the other hand, the HIS HD7750 cards I bought shortly AFTER I bought that Sapphire had all of their fans (except one) die in less than 6 months and that ONE didn't last a year - bloody junk "rifle bearing" junk.
Dual ball bearing fans DO tend to last a lot longer in high-load high-temp environments than anything else though, which is why I try to avoid any other fan type on my gear (and refuse to even CONSIDER the new Seasonic "Prime" and EVGA "G3" power supply lines).
Adding oil is at best a VERY SHORT term cure - if it leaked out in the first place, the seals are shot and adding more isn't going to last long.