My family's old Subaru (1990) has flat rocker arms. Believe me, I had to replace one when it broke

, but that's another story. I'm using Rotella T6 in it, and it seems to like it somewhat. Of course, it needed an oil change. The thing has always had trouble with noisy lifters, perhaps even when it was young. I have been wondering if that engine doesn't need some ZDDP, especially to protect against the intermittently clacking hydraulic lifters.
If only I didn't have to worry about that darned cat!