I have seen a plastic doodad that goes on a filter housing on a diesel engine which indicates when it is time to change the filter.
I have one laying around somewhere in my shop for some reason. Probably stole it from a place that fired me for no reason (or fired me for stealing small plastic doodads, i'm not sure)

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission