Originally Posted By: Impatient
Lazarus bump from the dead: if your car calls for 5w30 SN, maybe even 5w30 RC, is there any reason to prefer the 0w40over the 0w30...assuming you can get either for approx the same price (its been more effort, 5 bottles vs one, with rebate required, but doable) . Out of warranty: currently 123k mi.
My takeaway has been 0w40 SN is no more catalyst-friendly or “resource-conserving” than the 0w30 SL? If it matters, I’m talking about a 2007 turbo Subaru. Fairly short OCI’s (4500) but longer than the recommended 3750 mi. The oil “sheared” to 11.05 (the horror) according to the one Blackstone report I ordered a couple changes back.
Of course it’s now 2018, so maybe there are better options.
The Castrol has made motor more quiet (less metallic) than M1 5w30 / older 0w40 mix (mea culpa), certainly better than pure 5w30 M1 or PP.
Your car actually allows 10W-40 down to -0F. Newer VOA's on this oil show that phosphorus has been reduced further so in theory it should be even more cat friendly.
https://bobistheoilguy.com/forums/ubbthreads.php/topics/4510746/Re:_VOA_Castrol_Edge_with_Ti_-
OTOH, these cars sometimes throw wonky cat codes and replacing O2 sensors and cats don't always fix the problem and this happens on cars that have had a steady diet of GF-5 oil.