15W-40 HDEO is fine in an old BMW, 5T.
I ran a couple of changes of 15W-40 Tection Extra in mine last summer.
I will tell you that fuel economy is much better on M1 15W-50, though.
Enough so to actually pay for the marginal cost of the oil over a 5K OCI and then some.
A 325 e30 is a neat car, and pretty quick.
How many miles do you have on it?
Anyway, the Subie will be happy on anything, and it doesn't need 15W-40, although you could use it within Subaru's grade recommendations.
Fuel economy is bad enough on a starburst 5W-30.