Is there any downside to running a smaller oil filter like Wix 51085 vs the larger Wix 51515

I don't think so if you are using light oil like a 5-30 or 10-40. Now if you are still using a heavy 20-50 type oil then I could se it would have a herd time with the smaller pleats in the filter medium IMO.

Gus

I haven't had or seen any problems since changing to TTI headers and using the 51085 filter from before using a 90 degree adaptor and a 51515 ... I use 10W30 in my 410 c.i. small block

Here is a long discussion on this:

https://bobistheoilguy.com/forums/threads/small-vs-larger-oil-filters.307682

One downside to smaller filters: Less filter media means a greater pressure differential before and after the media. This will cause the filter’s internal bypass to open sooner when the conditions are right (colder higher viscosity oil, high flow rates due to higher rpm).
I personally don’t like the idea of open bypass, but they are necessary.

I run the super small Mopar filter on my stuff. The small filter fills up sooner so you get oil pressure sooner and it takes up a lot less space. If you're worried about it just change it more often.

Those are non issues on a big block with a preluber like an Accusump, which in my humble opinion is the single best lubrication addition for those of us that only occasionally drive our cars.

I've heard from a couple of engine & mechanical gurus that a small filter runs in bypass more than a larger one; especially at higher pressures.
On a couple engines I've run the 2-quart GM truck filter, but for my BB Mopars the old-school 1-quart.
As for fill time, I pre-fill them anyway - 1/2 way on horizontal filters, full on vertical ones - and there's always oil in them when I change them out.

I run the same filter as Andy, the real small one. Been using them on my blown 340 for over 25 years. It's only been apart once but everything looked great.
Doug

I will throw this out there as far as oil filters go. On a performance engine I like the system one oil filters. Yes, they are expensive, but eventually they will pay for themselves as you just clean the filter element. two they are built strong enough that you will never blow one open, seen that happen on regular filter a few times. three if you have any issues with the engine really easy to pop off the cover and look for metal, yes you can cut open a stock type filter, but it is a mess and difficult. They have several grades of filter media to get the protection you want.

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