The old advice was that the diesel oils had a higher concentration of ZDDP. Now with the advent of clean running diesels that additive has been sharply reduced and modified. Diesel has a different additive package and using it in a gasoline engine will break it down faster. Not a big problem if you keep up with regular oil changes.
Glad to see you are using a 10w-30. That is what the factory specs out. 10W-40 has a tendency to leave deposits in the ring lands over time and creates extra stress on the cam gear.
First choice would be Brad Penn 10w-30 or even 5W-30 when it gets cold. The BP oil has all the additives to prevent cam wear and is the closest oil your going to find that matches up with what the designers of the I-6 engines spec-ed for oil.
I use BP 10W-30 in my stroker and would not consider anything else.
Hmm might just put in some Dino oil, idk if my seals could take synthetic
Rotella is a great oil for spark engines as well as diesel. Suspends particulants well so that the filter can pick them up. It is a dino oil, not synthetic. I have been using it in my diesels as well spark engines for decades and have never had any oil related problems whatsoever.
I just did an oil change I had out delo in it and you'd be amazed at what came out, oil was blacker than all hell, and had random sludge pieces fall out, cleaned it up pretty good, and now I put valvoline 20/50 in it and it seems to run better with thicker oil in mine atleast...