Not seeing oil on ground but too into shop for something else and told this needs replacement. Would it only leak when driving?

Only as it get up to pressure .

The "oil level" sender is bolted tot he side of the oil pan with two bolts and has one electrical connection. The leak usually comes from a worn out $2 gasket. You could easily replace this gasket yourself. Make sure to clean out the sender unit with some degreaser to make sure the float inside the chamber is as clean as it can be from all the old oil.

As an abundance of caution, I would replace the whole unit with new (comes with a new gasket) and likely never have to do this again. If your shop won't charge you more than $50 for everything, then have them do it.

The "oil pressure" sender is different and is threaded onto the oil tube just below the oil filter. It looks like a small flying saucer on its side with one electrical connection . It's also and easy and cheap DIY job.

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03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio

92babyblue, the oil level sender would leak all the time since it senses the oil level in the oil pan, and is below the proper oil level. As they age the oil level senders tend to leak at the electrical connection. If you are going to change it yourself (as mentioned a simple DIY job) do it when changing oil, i.e. with the oil drained from the oil pan ... or you will have oil coming out of the hole in the left side the oil pan. Be sure to use a new gasket with the new oil level sender.

Normally the oil pan is not pressurized when the engine is running, unless the engine has worn piston rings that allow blow-by.

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Fred Hoelzle