Yes, you most definitely must filter it.

I used to bother with pouring my drained oil through several layers of muslin, then pouring it in the tank, no more than about two quarts per tankful. It worked great, but was just too much of a hassle to bother with.

A major rental truck company filters drained oil then pours it in the fuel tanks. The difference here is that even five gallons or so of filtered, drained oil is dilluted well in the 250 gallon tanks.

Have a great day,