So I have a 2004 g35 sedan 157,000 miles automatic. I live in Chicago, car recommends 5w30 I’ve been using valvoline high mileage full synthetic. Was thinking about switching to mobile 1 or royal purple. Should I keep using synthetic ? Should I go from 5w30 to a 0w ?? What would be the best for Chicago whether. It is fall now and temps are usually 60- 80 degrees outside for another month or so then it will be getting cold

 

Don't switch to synthetic if the car hasn't had a history of using it before. If you plan to change oil every 3750 miles, high mileage conventional is fine. Use the weight based on the temperature chart and predicted driving seasons in your owner's manual.

Best oil advice anyone has ever given me: "What kind of oil do you recommend? Clean oil and topped off."

 

That's on old wives (or mechanics) tale. Nothing wrong with switching to synthetic. Did that in my Accord, bought it with 75k miles on it, previous owner used regular oil. I switched to Penzoil Platinum Plus. Currently at 259k miles, zero leaks, oil consumption is less than a half quarter per 7500 miles.

But clean and topped off is fine. Biggest advantage of synthetics is they lubricate better on cold starts and you do go further between oil changes.

 

Either way, run synthetic if you plan to change at 5k mile intervals or run conventional at 3.75k mile intervals. Modern conventional oil is quite good but synthetic is still superior. My old 4ages seemed to leak much worse with synthetic, but that was probably just an illusion, no quantitative basis to it.

Reason I still end up running conventional and not just synthetic is the availability of high mileage formulas are much more common in conventional.

 

5W-30 in Chicago winters, no harm in trying out synthetic but you need to check your oil weekly and if it has a worse consumption problem on the synthetic then change back to conventional on the next oil change.

The myth about not being able to go back to conventional if you have ever used synthetic is 100% complete bullsh!t.