On this daf, we find a halacha regarding chatzitzah when immersing a vessel in a mikvah.
One man who wished to convert was slated to go to the mikvah when he was told that he had a problem. He had been forced to change the oil in his car and his hands were stained. When he got to the mikvah, his bais din pointed out that this might well be problematic: “Oil and the like can block an immersion from taking effect. The halacha is that if one is particular to remove a substance, even if it is on only a small part of his body, this also blocks the immersion from taking effect. It is only if you truly don’t care that this substance is on you that it does not block the immersion if it covers a minority of the body.
“But I often change the oil of my car—I am a mechanic—and I am not particular about this,” the convert-to-be said.
The rabbonim explained that it was still not so simple: “Nevertheless, if the oil can be removed, the custom is to remove it. It is only permitted if the substance cannot be removed or if one immersed without removing the substance post facto” (Sefer Haterumah, #79; Semag, Asei’in, #27; Mordechai, Shevuos, #648; Chochmas Adam).