I've been building Diesels and maintaining them for years, spend the couple of extra bucks for the Rotella syn, do not extend the oil change
intervals, at least not the hourly ones, calendar maybe. Reason is it's extremely uncommon for Diesel engine oil to suffer any significant breakdown as long as it's changed on schedule. You change Diesel oil when it essentially gets so loaded with contaminates that it needs it, not because it's breaking down. I feel pretty sure the recommendation for straight 30W is for people that don't change their oil when they should. The further the spread in viscosity for a regular oil, the more it depends on it's additive package to get that, the additive package is what breaks down first, causing oil that is overdue it's change by a wide margin to lose it's viscosity. Synthetic oils commonly don't need additives to have a wide spread in temperature range, hence no additive package to break down. It's only one reason sometimes synthetics are superior. Having said all of this, if you maintain your engine like you should, run it like it should be run, change oil regularly etc. I think you may go as far as 10,000 hours on an engine running regular oil, just Syn give you a little cushion for those times when you should change your oil, but can't, plus I'm convinced from engine teardowns that it does cause less wear, but some of this may be due to the fact that a person that will spend more on their oil, may be more likely to properly care for an engine than someone that doesn't.