Does your heating oil tank have an OSV?
 

http://www.cernakfuel.com/images/osv.jpg

www.websterfuelpumps.com/pdffiles/osv1.pdf

I'm thinking of installing one, but it seems like it should be installed after the filter.

Filtered oil would be less likely to have floating particles (crud) that could
jam up the valve's operating surfaces and render it useless..

If you have an OSV, please check out the installed location and let me know.

Thanks,
Rich

While I don't use oil, never have and never will, I have seen about a dozen oil tanks in the past month or so with those. The ones I have seen in recent memory are usually after the filter but not always. One I recall, which was after the filter, was just installed. The old one was dug up from the ground (some code violation or something) and replaced with a new one in the garage.