Is there any downside to running a smaller oil filter like Wix 51085 vs the larger Wix 51515
Yes, don't do it unless you have to. Why run a smaller filter when you don't need to?
I've tested different oil filters on different engine dyno years ago in SO CA on "The Carb Shop" engine dyno both stock and then two remote mounted stock type and then four remote mounted stock type and size. No gain or loss on using multiple filters, the smaller filters have less filter media in them so they have to bypass more oil to flow the same volume of oil than the larger filters due
Unfiltered oil isn't as clean as filter oil is, correct
I use one of the much larger size NASCAR size remote mounted oil filters now
IHTHs