Just want to throw in a comment since I have a high mileage S1 L67 as well:

I'm not sure what Bill B. used when he owned the car, but I've only used Mobil 1 fully synthetic since I bought it from him (2 oil changes now) and I've lost negligable amounts of oil in both changes. That included letting the system drain bone dry last change (aside from whatever was at the bottom of the pan).

Bob'* right. 12 psi at hot idle, if correct, likely indicates wear in the engine - probably the bearings. If you can afford to do it, you might want to plan on a rebuild or a lower miles engine from the junkyard.

If you cannot afford it, you may get by for a while, even quite a while using a heavier oil. Generally speaking, all the vehicles I have bought (approaching 150 now) have had 100K mi and more on the odometer. For those with lower oil pressures, I usually change the oil to 15W 40 Shell Rotella and will sometimes add a bottle of STP to improve film retention for cold starts. If your bearing clearance is getting up there, thicker oil will definitely increase the oil pressure and will do a better job of protecting your bearings. Heavy, straight weight oils provide great film strength. Racers who run hot and put great demands on their engines will use straight 50 weight oils. Valvoline makes a good straight 50 wt. "racing" oil. Heavy oil and old loose engines are made for each other.

From my experience, it will.

However, before you do anything at all - you might want to check to make sure that your oil is not being diluted with gasoline from a dripping injector. Gasoline dilution will lower the pressure and destroy the lubricating properties of the oil.

Hmm, how can I tell if an injector is leaking?

Smell the oil...if it smells like fuel..then one is probably leaking.

That'll **** me off, I just had them rebuilt a year and a half ago.

Noting that he lives in Illinois, if he goes to straight 40 or 50 weight, he'd better get a block heater or a heated dipstick for that car, or it will hammer badly on cold startup, which is when most bearing wear occurs.

Also, since rod bearings wear more than mains and certainly cause more oil loss when worn, throwing in a set of .001 over rod bearings )which can be done from from underneath with (the engine in the car) might be a fairly cheap solution.

But if it has to come out, it'* rebuild time.

15W-40 would be safe without a block heater, then?

Yes, easily.

Put a mechanical gauge on it and go from there...

Only thing that'* not mentioned here, suprisingly is you may have a weak and failing oil pump, in which case, running the heavier oil could help it die....Just a thought.

And you'll be fine with 15-40 oil until late fall.....or when the temps hit about 30-35 over night.