What we don't know, is how much different the FA24DIT is from the FA20DIT. On paper, the 24 is simply a bored-out 20. I've thought about this for a little while and, while I'm no expert on either engine, I'd be curious about the following before I could give a really adequate answer:
1. Bearings: IF all of the internal bearings are the same I'd be apprehensive using XW-20.
-----HOWEVER, we know that Subaru's 5W-30 syn shears to a 20wt quickly, so there are likely a good portion of FXT and WRX owners effectively running around with a 20wt anyway...
2. Octane: We know that the FA20DIT was tuned from the factory to utilize 93 AKI fuel. The FA24DIT was tuned for 87 AKI and won't benefit from higher octane w/ out an aftermarket tune. It has to be true that this engine can run regular fuel in large part due to the ECM's fueling scheme. I'd like to know whether this results in a substantial increase in fuel dilution for those who like to stay in boost often.
3. Power output between the two engines is very close (not that PEAK numbers really tell the whole story):
-----HP is only 4% higher and Tq is only 7% higher with the FA24DIT, which
-----I'm not saying a 20% increase in displacement will result in an equivalent increase in output, just that it doesn't appear that Subaru is really pushing this engine to the limit.
Conclusions:
1. Subaru seems to like to play things relatively safe and, based on the FA20DIT being around for so long leads me to believe that they've had plenty of time to carry over any lessons learned and improve on any design deficiencies to make the FA20DIT a pretty robust offering.
I think it's safe to say that following the owner's manual will VERY likely result in many years of good service
2. WITH ALL OF THAT SAID: I was a guinea pig for Subaru when I purchased my FA20DIT-powered '14 Forester XT. Many of us, myself included, experienced a ton of teething issues that resulted in inspections and half a dozen ECM flashes to resolve the many issues. Because of this, I would never buy a vehicle with an engine less than a few years old from Subaru again. While everything is running smoothly now, it took years to get things settled, so regardless of EVERYTHING I typed above, I'd be wary of this new engine...for now.
The engine has been in the Ascent for a couple of years, has spread to two other models (if you count the Legacy and Outback as different enough to warrant comparison, based on their history on the same platform and now that everything is on the Subaru Global Platform (SGP), and will likely spread further (can we all say together, "400HP/Tq STI, please, Subaru!!!"?