oily substance

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Leave it in place and clean it off really well and dry it well. Use some toilet paper to press against it checking for wet spots on the toilet paper. This way you will be able to tell if it is coming from the threads into the tank or the valve itself.
If and when you take the valve out of the tank be sure to have a small catch container with you as it is common for a few cups of gas to come out as the valve is removed.