Your oil pressure gauge readings sound normal, the needle should peg above idle and when hot should be around the mid point, i.e., about horzontal. Your leak may be due to other gaskets having shrunk over time. You might try snugging up the sump and cranckcase bolts, use care and do not over-tighten, just make sure nothing is loose. Thoroughly clean the engine top and bottom, some of your leak may be residual oil from the valve cover leak. One-half quart in 1,500 miles is not bad at all. An old friend and sage mechanic once told me that leaking a drop every 100 feet will add up to a quart in a 1,000 miles. Don't know if the math works out, but sounds like its a toss-up for you whether you are leaking more than your engine is burning. Also, remember that some oil always gets burned, if not, then something important is not getting lubricated.

Have your cold start and aux. air systems checked. If it runs OK when warm then they may be acting up. You also should check that all the vacuum hoses are tight and not leaking; vacuum leaks are very bad news and will cause all manner of problems that prevent good running on an otherwise fine engine.

Finally, the beastie may just need a good tune-up and some TLC from its new owner. I suggest giving it an affectionate nickname and always speaking nicely to it and about it when talking with friends. Cars are very perceptive creatures and can sense fear. When they do, your wallet is not safe...

Be of good cheer and welcome to the great society of SL owners.

230/8