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Shell Rotella, Delvac etc, brand 15-40 or straight 30 weight is probably the best choice. But if you know how to read the label other brands may be cheaper: Traveler, NAPA, WalMart SuperTech.

ANY heavy duty diesel oil will work. You REALLY want an API CF, CG-4 or CH-4 spec oil... or the European equivalent.

The reason is that most 'S' API rated oils SJ, SL, SM have virtually eliminated the ZINC flat tappet valve lifters need for lubrication. Since passenger cars have bits that can be damaged by zinc in the exhaust (O2 sensors and Catalytic converts) API 'S' (passenger car/gasoline engines) have been much quicker to remove zinc than the diesel oils.

You can use API 'S' rated oils but I would recommend a zinc additive (google zinc oil additive) and it generally abbreviated ZDDP. A bottle can be $10-15... so its much cheaper to use diesel engine oils.

GMC 270's are tough motors, but they most likely are going to be pretty worn.... and 270 parts ain't cheap.

Yes, our intrepid leader Joel Gopan did recommend it eons ago. I had trouble with it... had a smokey engine.... but after 2 oil changes the CCKW likes it again... it must cleaned up some crud???

Here is what Valvoline has to say about zinc in oil:

http://www.valvoline.com/faqs/motor-oil/racing-oil/

What is motor oil with zinc?

The anti-wear additive simply referred to as zinc by most car enthusiasts is actually short for Zinc DialkylDithiophosphates or ZDDP. Its primary role is to prevent metal-to-metal contact between engine parts by forming a protective film. Despite being referred to as zinc, this additive actually contains zinc and phosphorus, with phosphorus performing the anti-wear function in the motor oil with zinc.

Why is it important to have the zinc/phosphorus levels in motor oil changed?

With ever increasing limits on emissions, automobile manufacturers have tightened emission control systems on newer vehicles. This is one of several factors considered when the American Petroleum Institute (API) sets standards for motor oil with zinc. The current API standard is SM which replaced the previous SL classification. Because phosphorus can poison a vehicle's emission system, the level of zinc is lower for current motor oil.


What is the controversy surrounding the amount of zinc in motor oil?

Many hands-on car enthusiasts and engine experts believe the lower levels of zinc in SM motor oil is causing excessive wear in older style push-rod and flat-tappet engines. This is despite the fact that all new motor oil classifications are intended to be backward compatible. This has resulted in the widely accepted belief that modern motor oil is not adequate to protect older engines.

"Quaker State engineer Mark Ferner said zinc's extreme pressure qualities also aid in reducing piston ring-to-cylinder bore wear at BDC and TDC, where the rotating assembly sees an abrupt change in velocity and direction. Besides its extreme pressure protection, multifunctional zinc also functions as an antioxidant. That is, things exposed to heat and oxygen are subject to oxidation. In the case of a ferrous metal like iron, oxidation causes corrosion, which we see as rust. As applied to motor oil, Ferner said as the "heat accelerates oxygen, the oil starts to oxidize and thicken, changing into other molecular combinations. As the oil thickens, it forms deposits or varnish. In motor oil, the antioxidant additives first sacrifice themselves to prevent this oxidation from occurring, but if they get overwhelmed, the oil eventually turns into a hard, crusty sludge."

In its antioxidant role, zinc fights heat buildup in various hot spots like the piston-to-piston ring interface and the underside of the piston itself. Oil can accumulate in the honing crosshatch marks in the cylinder bore where it is exposed to combustion temperatures as high as 400-450 degrees F, a prime candidate for oxidation if not protected. Today's modern reduced-zinc passenger-car street oils contain other dedicated antioxidants to make up for the loss of zinc.

The potential zinc downside is that in theory, an engine with excessive blow-by could accumulate zinc deposits in the combustion chamber, potentially becoming a detonation magnet. But Ferner maintained, "You would need a fairly good amount of blow-by to really be bad for the chamber. There used to be 1,400-1,500 zinc ppm in [passenger-car] motor oils, about where [Shell] Rotella [diesel-truck] oil is now. We ran those high levels for years and years and never had a problem with chamber deposits building up. Zinc accumulation mainly affects oxygen sensors and catalytic converters, which are very sensitive to the ash that may accumulate on them."

Read more: http://www.hotrod.com/pitstop/hrdp_0702 ... z1sq2BAt3l

"There used to be 1,400-1,500 zinc ppm in [passenger-car] motor oils, about where [Shell] Rotella [diesel-truck] oil is now."

'Zinc accumulation mainly affects oxygen sensors and catalytic converters, which are very sensitive to the ash that may accumulate on them."

Read more: http://www.hotrod.com/pitstop/hrdp_0...#ixzz1sq2uZ6Ss

Mobil says:

". For products that meet the new ILSAC GF-5 specification the phosphorus levels for the oil must be less than 800 ppm phosphorus. The ILSAC level for phosphorus has been reduced to protect the catalytic converter and other emission protection equipment. The engine manufacturers are confident that this level of phosphorus will protect both new and older engines"

http://www.mobiloil.com/USA-English/...otor_Oils.aspx

So a car oil has roughly HALF AS MUCH of that good zinc/phosphorus we need.

That's why its important to look for that CF, CG-4 or CH-4 spec on the label.