Martin, I switched to Delo 400 a couple of years ago because of the detergent qualities. Your fuel sender needs to be pulled and cleaned. That is a common problem. There should be info under "search". Don't know what to say on the noise. Do you have a "indie" picked out locally for help? Search the forum for recommendations.
The drive belt isn't making noise, it is probably the idler pulley. Change it out with a MB replacement. And the plastic chain guides should be replaced, at least the uppers. Pull a valve cover and look. If they are dark colored it is probably time. If one breaks, kiss your engine "adios"
Anziani
'97 CL600 52K