I had M rods in the MPPE, because they are much stronger than the B20 8-bolt rods, and because they use the same wrist pins as B21 pistons, which was the cheap way to have a large overbore.

Later, I found out I could use 6-bolt rods modified to accept the B21 wrist pins, so I built a few engines that way. Then B21 pistons became very difficult to find, so I went with having custom pistons made in larger bores to fit the 6-bolt rods. Those are actually lighter than stock-bore Mahle pistons, but it does cost a bit more.