Runs as cool with the 50wt as it does with Synthetic, at 158-168* oil temp on even hot (90* + days) in moderate traffic conditions.
I do not think this is good news. I think it means a dipstick mounted temp probe, which I assume is what you have, is not giving you the information you want. I'll bet the oil in the return line is a
whole bunch hotter than that. The important temperature we wish we knew, in order to feel good about the oil's capability, is the temp when and where the oil is doing it's job.
On an air cooled engine on a hot day, especially in traffic, I can't believe that there is ANY part of the engine that is not much hotter than that. I'll bet the oil passing thru the cylinder heads is a getting to well over 250 degrees, so if your temp probe is on the dipstick cap, I'd be afraid that means that there is so little oil circulation, that even if the oil coming back from the engine is boiling, the oil reservoir never gets that hot.
Even if you had an oil cooler, I would think that if you were going to measure the oil temp, to be sure it can take the conditions, it should be measured just as it exits from the engine. You are very comfortable with your grade of oil, because of that low reading of under 170, what would you think if the oil coming out of the engine was actually 340 ?