Oil Viscosity question

[Guest ReattaRobert]

What are your thoughts as to why the 'C' engine has as its 'Preferred' oil - SAE 5W-30 whereas the 'L' engine sez - SAE 10W-30 is its 'Preferred'? :confused:

[2seater]

That seems odd? The trend is to lighter weight oils with tighter internal engine clearances for efficiency and emissions. The two engines are essentially identical, at least the early "L" engines are exactly the same, except for how the cam is retained. What is the source for the information?

[Guest ReattaRobert]

No lesser than the 88 GM FSM and the 91 GM FSM. The 88 Owners Manual agrees with the 88 FSM but I don't, as yet, have a 91 OM. However, I would be very surprised if it were not in agreement also.

Each of the above references does show the same three recommendations, i.e. 5W-30, 10W-30 and 30. My point is: Why the change in the 'Preferred' recommendation?

And, yes, I agree with your observation relative to lighter oils and tighter manufacturing tolerances.

An interesting, slightly off Reatta comment on my part is that Chevrolet with their vaulted LT1 engines in both Public Safety and Impala SS applications recommend SAE 5W-30. And those animals are good for north of 140! :cool:

Edited March 9, 2010 by ReattaRobert
spelling (see edit history)

[BrentS]

And, the "W" number is mainly for the viscosity rating of the oil at cold temps, and therefore both oils have the same viscosity at normal operating temps.......30.....therefore, "they" wanted a different cold-temp viscosity...........for what???

Some wrench-brain must have the answer...

[padgett]

I have used 10W-30 all along but am in a warm climate. My understanding is that oil companies use long chain polymer additives to get the desired viscosity points and that over time/temperature they become short chain polymers and tend toward a straight weight in the middle.

My real question is whether High-Milage oils are worth an extra 75 cents a quart.

[2seater]

My 1990 FSM recommends the 5W-30 but the only real difference is the cold weather performance as mentioned. It is similarly illustrated in the FSM. I confess, I have no idea why the recommendation would change. It probably makes little difference if a summer only car. Usually there is a label under the hood as well. I presently have four difference vehicles, the oldest being my '90 Reatta and the newest is an '05 Chrysler, with a Ford truck and Saturn in between. All list 5W-30 as the recommended oil? Curious situation.

[Guest barra]

More of a question than a response: What effect does an oil additive like Lucas or STP have on viscosity? I use Lucas oil treatment and wonder if using it in cold Michigan winters is harmful?

[BrentS]

I either use mid-priced oil OR (depending on vehicle) spring for the FULL synthetic. I never bother with all the so-called "high mileage" or "part synthetic", because you never know what the difference is and/or never know what percent of the oil is synthetic.

Full sythetic definitely has higher tolerance for temp/time effects, as well as viscosity changes (i.e. increases). The aerospace industry loves full synthetic.....most of their fluids are (MIL-L-23699, etc)

However, if someone's nerdy about changing oil every 3000 miles anyway, the benefit of full synthetic may be negligable.

[padgett]

Keep in mind that mil spec is usually deep into the cold range (-55F) and max load. Syntetics make a lot of sense particularly in the arctic.

My cars experience something completely different, garaged tropical climate and optimised to run cooler than normal and engines like 90F 90% much better than 110F 0%.

Coisequently I use dino since change often but am not really cost sensitive to the difference between $9.50/5 qt for regular vs $12.50/5 qt for high milage if there is any advantage.

[NTX5467]

The reason for the variation from 5W-30 to 10W-30 probably has to do with the rated EPA fuel economy and emissions tests. What the manufacturers use for these tests is what they need to recommend for consumers to also use to maintain strict "as produced" performance, however minimal it might be.

STP is more of a viscosity improver (thickener) and people used to use it regularly with an oil change in the 1960-70s era. But if the engine was in good condition and the oil was a quality oil, it's real value in normal use was minimal. One time, we sold a crankshaft and bearings to a repair shop . . . the owner of the '76 Trans Am, with an engine that had a slight main bearing knock, had been using one can of STP to keep it quiet, so he changed the oil and added two cans, with each can allegedly upping the oil's viscosity number by "10", so with 20W-50 oil, he ended up with somewhat unpumpable 40W-70 oil. New crank and bearings fixed it.

Just some thoughts,

NTX5467

[Guest Richard D]

Living in Miami I follow Padgett's plan, on the daily driver Reatta I use Penzoil 10-30 dino oil and change it every 3,500-4,000 mile range. On the Skylark(350 2 bbl) that only gets about 25-30 miles a week gets changed every 500 miles or six months, same dino oil as Reatta.

Edited March 16, 2010 by Richard D (see edit history)

[padgett]

BTW on page 241 of the 1990 owner's manual it states that 10W-30 is OK if the temperature is going to be above 0F.

I doubt that there would be much difference warmed up in MPG between 5W-30 and 10W-30.

Do recall a Mfr in the '70s getting in trouble for using full synthetic "break in oil" for the tests but never using again.

[Guest ReattaRobert]

What this all boils down to is 'Why did Buick change the preferred oil viscosity'?

As I read the two charts more closely (88 vs 91), I notice that the stated temperature range for 10W-30 is the same on both charts BUT the range for 5W-30 changes from below -20 F to above +100 F (88) to below -20 F to (only) +60 F (91).

This would suggest that the L27 engines have looser manufacturing tolerances than the LN3's. I don't have access to the 89 or 90 info so am surmising that the vis - charts for them are the same as 88 being the same LN3 engines.

Tis a great mystery that quite likely remain so! :confused:

[charlie1]

I was told GM did that to increse the overall CAFE standard for EPA mileage. What seems like mouse nuts in increased mileage adds up when they take the total fleet of cars into consideration.

Edited March 17, 2010 by CHAS1 (see edit history)