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Wow, there sure are a lot of posts to this thread with no actual performance tips! OK, here goes:
1- As someone mentioned, make sure the trap oxidizer is removed. This picks up 5hp (more if it was plugged.) The dealer will replace it free as part of a factory recall, and also replace the turbo free if any damage is observed.
2- If the car is very sluggish off the line, you can try adjusting the ALDA to correct this (assuming the engine is otherwise in good condition.) This is not easy to do on an OM603, the ALDA has to be removed to access the adjustment setscrew. Post a new topic specifically on this if you want further help (I haven't done it personally, YET!) This will NOT increase top-end power, just the off-idle response, and only if it's poor in the first place. If it's good now and you adjust it, it will be worse, not better.
3- You can adjust the turbo wastegate to increase boost. HOWEVER, fisrt make sure it is reaching factory boost spec, which is 0.85-0.95 bar (13-14psi). Plumb in a pressure gauge with a "T" fitting, route the hose out the hood seam through the window, and go for a drive - max boost will be at full load & high RPM. If less than the above numbers, you're not getting what Mercedes intended. If you are, and want more, you can tweak it a LITTLE bit, say up to ~1.1 bar (~16psi). That's when the overboost sensor cuts in. Any more than that and you would have to disable the overboost safety, which is a bad idea, and you also risk serious engine damage. The extra HP also puts a strain on the transmission, so be warned if your tranny is old and stock...
4- A K&N air filter won't help at all. Save your money. Diesels run with an excess of air, more airflow won't help. Use an OEM (Hengst, Knecht, Mann) filter and make sure it's clean.
5- Reduced friction is always good. Use a top-quality diesel-rated synthetic oil. I use Mobil Delvac-1, which is a super-duty diesel version of Mobil-1. Most full synthetics are fine but per Dr. Booth's comments, stay away from Castrol Syntec. I found Shell Rotella Synthetic at the local Wal-Mart for $19/gallon, but haven't tried it yet. The price is pretty good though, for a diesel-specific synthetic, and a lot easier than getting Delvac-1 at the Mobil distributor!
So, yes you can get more power from your 6-cylinder OM603 engine, just not a lot more! BTW, my 1987 300D's have the same engine as your 300SDL (yours is an OM603.961, mine are OM603.960)
Good luck,
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