My 2020 owners manual states:
A. "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.
. When adding oil, you may use different brands together as long as they are the same API classification and SAE viscosity
as those recommended by SUBARU."
B. "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."
So I am confused by the lack of a clear direction from Subaru:
1. 5W-30 synthetic oil is recommended which I assume means full synthetic oil and then they switch categories.
2. 5W-40 conventional oil may be used if replenishment is needed.
a. Why did they switch between oil types?
b. Why do they exclude 5W-40 full synthetic?
c. How much "replenishment" is permitted? 1/2 quart, 1 quart, 3 quarts? Why is a maximum volume not defined?
and the key statement by Subaru which is the basis of my post is:
3. However, in hot weather, oil of higher viscosity is required to properly lubricate the engine.
How am I as an owner which reads their Subaru owners manual for their WRX interpret Subaru's own statement that a higher viscosity oil is required in hot weather?
Since my oil statement was the use of 5W-40 full synthetic oil vice the 5W-30 synthetic oil:
i. The "cold" weather "viscosity" rating is the same at "5W" for each and "cold oil" is significantly thicker than hot oil so "small oil passages" only come into play with cold oil.
ii. My alternate "hot oil" viscosity increase from "30" to "40" stays within Subaru's "approved" alternate viscosity.
I don't want to drive my WRX out of warranty period on 5W-30 oil only to find out my rod bearings crapped out with less than 200,000 miles on them because they should have been filled with 5W-40 oil?
I rest my case of confusion! LoL