What's the issue? It appears that unlike the Onan generator discussed recently, that they do spec synthetic oils.
Also, it would probably help to specify a year for this engine. There are synthetics designed for modern diesel emissions equipment. I'm not sure the same is true for conventional oils.
I once switched to 5-40W Shell Rotella full syn and when I adjusted my valves the engine head was really clean, but then I started to see more blow-by. So I switch to Chevron Delo 400 15-40W full syn and that's what I run now in both my generator and engine. I don't recommend 5-40W for warm weather driving. The 15-40W is best for spring -summer-fall driving above 40F.
I have have a C7 in my 2004 Itasca and use Shell Rotella T5 10w30 which is a blend. I change oil every 6 months or 3000 miles. One of the forums recommended 10w30 as being easier on the heuy pump. I test the oil once a year. So far so good.
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Dave T.
2004 MERDIAN 36G Freightliner & CAT C7 330
Retired LEO