I have been using the rotella synthetic 5-40 in all my stuff for 15 yrs or so
went to it just for the 5 in there for cold starts of my diesel van then liked the oil so well I have used nothing else for years now
I could start the old 7.3 at -30 without plugging it in not a really smart thing to do but just being able to have the capability of starting it was enough for me
I tried extending my oil changes with it for a while would slap in a new filter every 8000 miles and send in a sample for annalasys but after 32000 I decided it really should be changed even if the sample sheet said extend another 10000
If you started a old 7.3L at -30F using that oil you are/were WAY over max for the oil and I bet 100 to 1, you glowed it for some long length of time. And you are adding extra wear to your oil.
Problem with R5w-40 is the pouring temp is -36C and the pumping temp is way below that. I have tried R5w-40 at -20F and glowed tractor for 5 seconds and nope. That is my MAX for glowing since glows are more $$ then a better oil to help. I tried a synthetic oil (group 4/5) and same temp/and time and started...Not even talking about better UOA based on baselines.
And when you add a new filter you are adding new oil and that helps the oil. What is you miles per year? How many UOA have you do on the oil? Oil filter used? I could do on and on. But the old 7.3 (before Powerchoke) were VERY easy on oils...