A&Q about 350Z
Q:
I was wondering if it would be worthwile to switch an older engine (92 GM LHO, 150xxx miles) over to a synthetic like mobil 1 or royal purple?
A:
Its never too late to use a higher quality oil. The leakage issues you sometimes hear about might show up on your car. Engines which use cork, paper, and fiber gaskets tend to crack and dry over the years. The cracks fill up with gunk and don't leak very much, they just sorta seep. Switching to synthetic with its higher solubility can clean out that junk and reveal some pretty hefty leaks and/or oil consumption and burning. The good news is that if you switch and start getting leaking or smoking, you can always switch back to regular, use a couple applications of a stop-leak stuff, and you should be back where you started.
There is no issue with switching oils as far as the engine is concerned. Its the same oil molecule, just that one is created in a lab and one comes from dead dinosaurs. The ones created in the lab don't have any of the refinement impurities which makes them "better."
A:
And generally, synthetics can be run for much longer periods of time before needing to change it, which offsets the high inital cost of the oil itself. Hell, people interested are getting good oil analysis results back after TWENTY THOUSAND miles of runing 505.1 and such high grade oils in their cars. Neat stuff.