This is Subaru's current owner's manual for all turbos:
Quote:
In choosing an oil, you want the proper quality and viscosity, as well as one that will enhance fuel economy. Oils of lower
viscosity provide better fuel economy. However, in hot weather,
oil of higher viscosity is required to properly lubricate the
engine. The following table lists the recommended viscosities and applicable temperatures.
Quote:
5W-30 synthetic oil is the
required oil
for optimum engine performance and protection. Conventional oil may be used if synthetic oil is unavailable.
If 5W-30 synthetic oil is not available, 5W-40 conventional (
) oil may be used if replenishment is needed but should be changed to 5W-30 synthetic oil at the next oil change.
I have contacted Subaru of America several times about "5W-40 conventional" but haven't been able to get anywhere. Others have contacted them about the temperature chart showing 5W-40 but the language referring to 5W-40 for replenishment only but SoA generally responds with use 5W-30. Dealers and SoA typically do not have a comment on the language stating that thicker oil is required in high temps.
I used to put the blame for the confusion in Subaru's manuals on SoA, but was surprised to see that the German manuals now have the same language, albeit with the addition of allowing ACEA A3 oils. Of course, Subaru Japan allows 5W-40 in all turbo's, the H6, and the BRZ all which spec 0W20 and 5W30 in N. America.
I underlined and italicized "for optimum engine performance" above since I believe this a little different than just stating "5W-30 synthetic oil is the required engine oil" full stop.