The 1987 300D air filter canister pulls air in from the passenger side fender, which has an opening which goes back to allow air to enter at the front door jamb. It's nearly impossible for any water to enter this area, and the panels under the engine & tranny have nothing to do with it. Perhaps the plastic fender liner on the passenger side is damaged or missing, allowing water spray into the intake? That would be the first thing I would check.
BTW - while I don't have much opinion about fuel additives (except that they shouldn't be needed on a regular bases), I do have a strong opinion on oil additives: don't use them. Period. If you feel the need to clean out your engine, use Group IV/V synthetic oil (preferably 5W-40 or 15W-40) such as Mobil-1, Amsoil, or Red Line and don't worry about it. After about 10kmi of this stuff, your engine will be as clean as it's going to get internally.
On a side note, I forget if I already pointed you to this link, but you may want to check out this 1987 300D checklist if you haven't seen it -
click here to view the text file.
