I've already replaced the cap and gasket - someone else mentioned that as well.

I will be going to a higher viscosity oil next time around - had I known about it then I would have had it done this time. I also added a can of an oil additive which said that it would reduce oil consumption - whether tis true or not I don't know, but I just checked the level yesterday after almost 700 miles and it was still full as per the dipstick. Maybe the combination of the two (new cap/gasket and the additive) did the trick.

Someone else suggested the Valvoline "old engine" oil. Is that worth a try? The only experience that I've had with synthetic is with my old '77 VW rabbit - I tried some not long after I bought it in '78 and it went right through the car. Most expensive oil change I ever had. Went right back to Castrol 20-50 and had no trouble.

Sean

It has been said that synthetic oil will leak more than dino oil, especially in an older vehicle with old gaskets. Unless you can do the engine work yourself, having it done will exceed the cars value. You can use a lot of oil before you even approach the cost of repair. ( Don't let the enviromentalists see this.;)) If it is in otherwise excellent shape overall, and you plan to drive it for a lot of years; you can bite the bullet and have it repaired. Remember, it still will be worth more to you than someone else.:D

I'm by no means a chemical engineer, nor even a knowledgeable mechanic, but at 150K I'm defining my 560SL as "high mileage". Valvoline's PR for their MaxLife product talks to additives to condition seals and reduce consumption and deposits--all of which make sense to me.
As far as viscosity, I do have the owner's manual, and I stick with its recommendations for the temperatures involved, going toward the heavier of the options given.