Napa Full Synthetic Motor Oil 15/50 Energy conserving SM rated?

87 300SDL, PO stated he was using synthetic and I have read here that it makes it run quieter. Car runs well, unknown milage but likely pretty high.

I ask about this oil because I have 2 cases of it and the Napa guy says its cool for Diesel but no C rating on the bottle concerns me. Im reluctant and fighting the urge to just use Rotella, the same crap my truck happily thrives on. So I ask the following without causing a heated debate over correct brands and wt.

Does my 603 really bennifet from synthetic oil considering I dont mind changing every 3k or so, can I expect it to run smoother and quieter with snythetic?


If synthetic IS bennificial is SM rated ok, should I be looking for the CD rating or something else?

The threads I have read so far are avid and contain endorsements for specific brands and wts, usually in sharp contrast to other endorsements in the same thread. I only want to know if a full synthetic is really bennificial and if so is what I already have proper (SM?).

Thank you.

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87 300SDL (Silver/red)
08 VW GTI (Sold)
89 F250 5sp turbo diesel
00 Triumph Sprint ST (4sale)
00 Polaris Xplorer 400 4x4
09 Kawi KLR