I've run the Diamond XDL more than 45,000 miles, and only then changed it at 1.5% stretch because I wanted to: even the sprockets were still in good shape (I still have those, 30 years later, in the box). Oil it every 2nd tank of gas (moly-laced Kal-Gard lube in my case), wash it full force at the carwash whenever I wash the bike, then ride 5 miles to fling out the water before lubing it again, lube it warm, usually.
I've never seen anything outlast the Diamond XL or XDL. Even their modern version, the Powersport, only gives me about 25k-30k miles. Harlel shops today still carry the XDL...if you can afford their prices!