I’ve always thrown away the oil cap – a colleague explained to me what it was actually used for

How to use the oil cap correctly
Remove the cap carefully
When opening a new bottle of oil, you’ll notice that the opening is usually sealed with a small plastic cap. This cap is usually designed to come off easily. Instead of throwing it away, carefully remove it and save it, as it will come in handy.
Put the cap upside down on the mouth of the bottle
The trick is to turn this cap upside down and put it back on the mouth of the bottle, but this time upside down. Most of these caps are designed to fit perfectly this way, creating a nozzle that helps control the flow of oil.

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