I'm not sure what the "recommended" is for the ML320. I however swapped out my sensors last week @ 99780 miles. Based on everything I have read 02 sensor replacement should be at 100k miles for all vehicles. I had budgeted for this replacement and had found the O2 sensors on line for around $100 each at quite a few places. I was pleasantly suprised when I got all four for around $260.00

See

http://www.babcox.com/editorial/cm/cm40230.htm

for a good article on O2 sensor replacement.

I have noticed a highway milage gain of 1.2 MPG highway (70+mph). So based on current fuel prices at the $65.00 / ea I paid, they will pay for themselves in about 7 months.

However my idle seems to not be as smooth. I am wondering if I need to have the adaptive engine values zeroed out to compensate for the new O2 sensors.

I have also heard that other O2 sensors can be used, and in reality any 2 or 4 wire O2 sensor with the correct threads could be used, and wired in. since 2 wires are the preheater wires, An unheaded O2 sensor would just run a little rough at idle. And probably be OK after heating up. But with all the MAS/O2/electronics problems these ML's have I wouldn't take a chance on something screwing up the computer. He@### it seems that the electronics are so integrated, I'm afraid to use my cigar lighter for fear the rear window washer won't work