It's mostly condensation slime that catches in those filters. The 80 series guys have been doing this on mud for some time. Yeah, some oil does collect there, but most of the guys that post pictures show some light brown sludge in it that they dump at each fill up.
1978 13BT-40, 70 series mechanicals
1966 FJ45 lwb stock
2003 100 stock
1974V8J40, 1992FJ80, 1985FJ60, 1971FJ55, 1978FJ40, 1974FJ40, 1997 LX450 all long gone.