I know I may start a war here but:
I have run 10-40 Pennzoil since the 60s in my ACs. It was what I used in my vehicles, available anywhere, and usually on sale. Changing it after working ground and when it darkens. The one C was overhauled in '68 or '69 and is still doing well on the bottom end. Ringed it a couple of years ago, and a couple of valve jobs in those 30+ years. Oil preasure drop is not noticeable, even when I boil the radiator over. A few years ago I got a real bargan on some Valvoline oil so I figured, "Hey, name brand oil should be ok". Wrong! Every time that stuff got good and warm, my pressure dropped. To nothing. Changed back to P 10-40 and pressure was back to normal.Since that episode I have talked with other people that have had similar experience with different brands. Seems like some oils tend to thin down more with heat than others. Maybe straight weight oils change more than multiweight oils--I don't know. Just more noticable on tired old tractors with gauges. Idiot lights on most newer vehicles don't light up till oil pressure has dropped below 10psi. So we wouldn't realise how much thinning is happening with our everyday drivers. Just a word of caution to those people would add thikeners like 'Motor Honey' or 'STP' to try to help their tractor's oil preassure. I have trashed two crankshafts in tractors that I bought and tried to drive home in cold weather. The oil in them had been 'Doctored' and was too thick to circulate when cold. I don't say not to use thickeners, just that ONE can is enough, more might bite you. This is why I ALLWAYS drain, flush, and refill with new oil, before I attempt to run a new arrival.
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