K7, this bolt came loose and I tightened it but now

Just remove the bolt and reinstall again using a soft washer (copper or aluminum), no need to tight much, you should not have any leaks.
Before reinstalling the bolt with the new washer you have to remove the existing OEM washer which is a collapsible type of washer (same as the oil drain plug). This can be a little difficult sometimes, just use something with a sharp edge and take your time rotating the washer until it comes off the bolt.

Timing inspection plug. Those are notorious for misting oil. Replace the crush washer as suggested. The heads on those are surprisingly soft too, so not a bad idea to replace the plug occasionally too.