I have a '93 with 145k and it started losing oil just recently. It had been sitting for around five years in a garage in the south before I picked it up and started driving it. Naturally I began using synthetic when I began driving it, but no problems immediately appeared. Synthetic is just better and I prefer to use it. After about 10k on Mobil 1 10W-30 I found multiple oil leaks around the timing cover and cam seal area. Syn will cause old oil seals to leak, most likely by cleaning the sludge built up from years of dino oil in the engine. However after I fixed the leaks around the front of the engine I haven't experienced any oil use. So the real problem is not the oil, its those old seals. I prefer to fix the old oil seals instead of going the band-aid route and regressing back to dino oil. 3500mi oil changes are inconvenient, less efficient, and just plain old-fashioned. There are synthetic options for older engines, so you might try going that route, or just fix the leak(s). Ultimately the engine will be happier.

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'02 BMW 325i
'85 300D 450k
'93 190E 2.6 170k(killed by tree)
'08 Ducati Hypermotard 1100S 6k
'06 Ducati S2R800 14k(sold)