There is not supposed to be a seal there, and I suggest staying with 30wt or 10w30 oil. There was an 0-ring at the factory to prevent leakage during shipping, but it is not replaceable. follow the directions in the manual to lubricate it and you will be fine. If you do not yet have a manual, use the crank to turn it with the screw to the right side. fill it with oil to the hole, then turn it 3/4 turn till the screw hole id down and let it drain. there is a standpipe in it that drains it to the proper level. turn the screw hole to the right side and replace the screw.
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