Guy at the range showed me his piston-driven AR interior after ~500 rounds the oil in there was still clean. The bolt back was immaculate. The only mess was where the exhaust comes out under the hand guard and blowing on the scope lens.
He also said he'd taking it back out to experiment and going back to DI he noticed a definite decline in inaccuracy. So he put it back in. That is the other way around to what I would expect.