So, I buy this 1991 940 Turbo. It has 230,000 miles on it but runs fine. Sounds like a diesel but runs just fine. The guy before me says he uses 20/50 during the hot months but for December and January he goes back to 10w30 so the car is not so hard to crank with such thick oil. I'm a valvoline guy but I pop the hood and see a Castrol sticker. I don't know what to do. It was 85 degrees here today and I don't really want to put thick oil in the car for the summer. Does anyone have any ideas about changing brands, and/or viscosity?

 

My 1990 turbo owner's manual recommends a 20w oil right down to 20f.
No way I'd run a 10w- anything in one of these engines, it's far too light. Personally, I run a 20w50 all the way down to 10c(14f) and have no problems.
I would NEVER run a 10w-20 above 15 c(55f) in any older engine, and I'd never run that engine hard at any temperature with that weight oil.
Oh, you should get past brand loyalty for oils, they're all the same.

Regards, Andrew.

 

Personally, I've run 20W-50 Castrol in 4 cylinder n/a engines since the 70's. But, I've always lived south of the Mason-Dixon line. The only advantage to running a 10W oil in winter is for the momentary cold start in the morning. In bitter cold, a thinner oil can certainly assist the car to start a bit faster. The thicker oil would require you to idle the motor 10 minutes or so to warm the oil prior to driving as your o.p. will be considerably higher.
That having been said, I run 10W-40 Mobil 1 in my turbos, but my engines have all been rebuilt in the last 3 years; none of my motors have more than 10k. I find the thinner oil feeds better w/ turbo applications, but that's just me...I've actually had a high mileage turbo 760 STOP smoking when I went from 20W-50 to 10W-30...go figure.
With the miles on your motor and the way you describe it sounding, I think oil brands and viscosity is not a major issue. Should you rebuild the motor at some point then I would highly recommend synthetic over dino oil simply because it does not coke and block passages the way dino oil will.
And, there certainly are oils that are better than others...but you won't find them on the shelf at Autozone. And their use on a high mileage car with unknown history is a waste of money.

 

I'm in Louisville, KY. After having phoned around to all the instant oil change places, I came to the conclusion that a $50+ oil change was just too much to spend. Of course, they all have their special blends, their high mileage Super Max Life Non Sludge Bio Organic Magnetic oils. Did I mention Super? I think one kind of oil used Super twice in its name.

So, I bought four quarts of Castrol 20w50, a good filter and stopped at the dollar store for an aluminum turkey pan. Rolling around in the driveway under my car made me feel quite like a teenager again. My shoulder this morning makes me realize my true age, however.

 

Back in the day, when I was a teenager....my Dad always told me to use "the best oil and the cheapest gas". Gasoline today truly is a crap shoot. You may notice how rarely you see a gas tanker pulling a marked trailer. That's because Texaco is fueling Shell and Chevron. The big companys are so incestuous now that they all pump to each other. Off brand companys get left over loads. Back in the 90's, the EPA had oil companys reduce zinc/phosphorous (ZDDP) from motor oil that improved cling. Not as important on modern engines, but on my 73 144 with a flat tappet cam I need all the cling I can get. ZDDP is still found in higher levels in higher priced specialty oils like Royal Purple and Royal Purple XPR.

 

ZDDP does not improve "cling". ZDDP (zinc) is an anti scuff additive, it provides a very fine layer of a soft metal (zinc) over metal parts when teh oil film breaks down, as it is prone to in non roller cam engines on the cam lobes and other high pressure areas.
The EPA did not mandate it's removal, catalytic convertors were introduced by auto manufacturers as teh cheapest way to control certain polluting gases, and ZDDP was found to contaminate certain convertors, so the SAE wrote a spec to reduce anti scuff additives of the ZDDP type.
This was a global thing.

Regards, Andrew.

 

Why gosh Andrew, kinda splitting hairs there aren't you? I didn't say viscosity...I said cling. Rather a generic term to be sure; depositing a thin layer of sacrificial metal would seem to me to be "clinging" to me. I think if you do your research, that the reduction of ZDDP from 1600ppm to first 800 ppm and now less than 400 ppm was an economic issue AS A RESULT of the federal mandate that o.e. cats last 150,000 miles. It is true ZDDP has a detrimental effect on cats. But surely anyone who refers to Zinc Dialkyl-Dithio-Phosphate as merely zinc can allow a person a simplification or two.

 

I can remember those cans of oil that were paper, really, with the metal lids where you had to have a funnel after you poked those holes in both sides of the top. They'd be stacked up in the window of the local full service station. They'd be right there near the pump, too, like that scene from Steve Martin's "The Jerk" where somebody is shooting at him.

Around here you'd use the brand of oil you dad used, almost like religion or a political party. That religion stuff fades over time, by the way.

 

Yeah, using what the old man used is a great substitute for experience; something young men don't have when they leave home. Buying the same stuff as Dad worked well for me until I learned about improvements and such.

Sorry to hear that...for me it's always been like the ebb and flo of the tides.

 

You were dead wrong about what you referred to as "clinging". ZDDP does not allow any such properties in an oil whatsoever. It is a secondary lubricant, used only when oil film thickness breaks down and helps prevent hard metal to metal contact by allowing the zinc to sacrifice itself. I don't recall mentioning viscosity.

Regards, Andrew.