If you do a search on this forum, you'll find endless discussions on oil. Ultimately, you must make your own "informed" choice. I think 15W is a tad heavy, even for an older engine. I used to use 15W until I took the oil filter cartridge out on a 40 degree day and found the oil as thick as mud. I have switched to 5W40 synthetic blend and I've noticed that the engine doesn't "rattle" at start up nearly as long as it did with the heavier weight oil. That is, the oil pressure comes up faster with the lighter oil on a cold engine. My Sl300 has 100K miles and doesn't use any oil between 4000 mile changes.

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