If you put in enough sand to completely clog the filter, the oil will bypass and circulate unfiltered (and sandy) oil through the engine.
It wouldn’t take a lot of junk to completely clog one of those tiny filters they put on some cars these days.
Sand in the oil, well I have heard that oil filters when clogged are designed to break through (not the filter element itself, but a ‘burst valve’ of sorts at the far end of the filter), which would allow sand to circulate.
Another way to stop enemy warcraft using IC engines, if given the opportunity (have to keep this thread open you know 
), is to put uncooked rice in the cooling system.
Nope. I was just wondering how sugar destroyed the mechanism.