300 te- 1989.
While under the car yesterday, I noticed a slight orange-ing of the metal around the jack hole...I did not pull out the plug. Is there any "preventive" maintenece for this area or the entire rocker panel? On an older Volvo I had, I filled the rocker with 80wt oil, plugged the drains and let it sit there for a few days...I don't know if it did anything, but I felt better about it.
So, what are the ways to prevent rust?

I noticed a bit of rust around mine as well recently. I removed the plugs for each jack hole, dried out the hole and added some spray grease, then re-installed the plugs. I think moisture gets trapped in there, behing the plugs, and causes the rust. I'm hoping the grease will do the trick. Not a bad idea to remove them a few times a year and dry out the holes, either.

Likewise on my '89 230TE. MB in the UK won't sanction localised repair at their dealerships and say the way to go is to replace the entire sill.

My experience is that regardless of the oils or waxes you spray into the jacking point, they will continue to rot, though flooding the interior of the sills with waxoyl might help.

The most economical solution on a 15 year old car may well involve repairing and plating over the jacking points and relying on a trolley jack for those flat tyre occasions.

One of my 190E 2.3's had this problem. I took it to a body shop and they grafted a new bit in and then "blacked" the rocker out as it was much cheaper than painting etc. It was about $200 for both sides. The car had about 250K on at that time. The "new" owner is having the problem come back now, but the repair lasted nearly 10 years and another 250K.