I know there's lots of engine lube knowledge here, so here's my quandry......

Scenario: 1978 w/454, just turned 90k miles, normal operating environment 40-90+ degrees.
First owner ('78-'03) kept meticulous records, showing oil & tall filter changes every 3K miles, using Pennzoil straight 30W....I have maintained that ritual...consumption (no leaks) is about 3/4 of a quart between changes. I also have a MMO inverse oiler system installed, which sucks 2-3 drops a minute into the carb at high vacuum and about 6-7 drops/min below about 5" MP. Never had
an engine lube issue I'm aware of......

I recently flipped thru the book, The RV Handbook, copyright 2000, by Bill Estes, which says in part:

SAE 30 , which was the preferred oil for GM's 7.4 litre engine in the company's motorhome chassis for many years, is no longer listed as suitable.

GM light truck manuals specify SAE 5W-50 as preferred and 10-W-30 as acceptable. GM recommends against using any other grades of oil, specifically 20W-50.

Question: Should I just leave well-enough alone at this stage in its life, or is there any derived benefit to be had from making a switch-over now?

Any & all theories appreciated........Thanks.

More important than the SAE viscosity grade is the API grade - IIRC, SE would have been current around that time, and SAE 30 was the usual recommended viscosity, with 10W/30 if hard starting was experienced in cold weather. In the '60s and '70s, I can recall no truck engine builder recommending a steady diet of multi-grade.

The reason the recommendations have changed over the years is, IMHO, the creation of viscosity index improvers (VIIs) that are shear-stable.

Multigrade oils start out with a base stock of the winter number ("10W"), to which a VII is added to make the oil thin less as the temperature increase, thus keeping it within specs for the second SAE viscosity ("30").

A VII is nothing more than a molecule that is coiled, curled, furled, or otherwise compact at winter temps, but uncoils at the summer temps. In other words, cold it acts small, hot it acts big. Since viscosity increases with the size of the molecule, bigger keeps the oil from thinning out as much as it would otherwise.

Back to the "shear-stable" thingy. Early VIIs were relatively fragile, and the molecules when uncoiled would be broken, or "sheared" mechanically by engine parts, and the oil thinned more and more as time went on. STP (and other brands using polyisobutylene) was an example of the worst

Until recently ('80s), VIIs still were subject to shear, but newer ones were eventually developed that don't break down, and so are "shear-Stable".

The bottom line is that either 30W or one of the recommended multigrades would do just fine, with the scales tipped in favor of the multigrades as they generally have a more robust additive package because all the premium motor oils are multigrade. My own personal opinion would be to use 10W/40, as SAE 50 may be too viscous. What folks sometimes don't realize is that a shaft spinning at high RPM actually is easier to lubricate - doesn't need as viscous a lubricant - as one at low speed.

quote:

switch

I live in the southwest and have run 20-50w in many engines over many years with good results,
I think the trend to lighter weight oils is mileage driven having little or nothing to do with
lubrication. Your 30w is likely fine but modern multigrades will perform better over a wider temperature range.

Just a side note to the shearing issue. Detroit Diesel to this day does not recommend multi-grade for the two stroke engines due to shearing. Straight 40 with the latest C?-2 rating is still the only recommended oil.

I use this Amsoil product because it has zink that all oil used to have back in the day. They say zink is needed to properly lubricate the flat tappets used on these older engines. I dont know if this is true but I am a Amsoil dealer so I use it.

Neil

quote:

They say zink is needed to properly lubricate the flat tappets used on these older engines.

Yea, I have heard/read that too Neil....'Course, I'm not lube-savvy, so I don't have a clue....

Rusty, what say you about that?..........

Zinc diorgano-dithiophosphate was widely used in the '60s and '70s as one effective anti-scuff additive; so was tricresylphosphate (TCP, also used in gaso as a lead-scavenging agent). The anti-scuffing tests are run on actual engines (back then, the Chrysler 383).

API generally requires specs to be backward-compatible, however, with the now-obsolete SH (and prior), the API requirement for zinc and phosphorous ended. Oil with zinc compounds is still recommended for use in pre-'90 engines, which may require the tappet lubrication Neil mentioned. SF/SG is still in use for motorcycles with wet clutches.

While contemporary oils may (actually, probably do) offer the same anti-scuff protection, use of SF/SG/SH ensures that "old-school" protection is included. An oil may claim multiple classes, such as SH/SJ/SL/SM, if it passes all the tests. Because each clasification requires multiple tests, and each VERY expensive, multiple classifications probably come from an oil meeting one spec then also meeting the new spec with little or no reformulation.

Anyway, I'd use (and do use) Shell Rotella T 15W/40 in my Ford 6.9L NA'87 van and '92-era 6.5L TD.

quote:

Originally posted by Rusty:


Anyway, I'd use (and do use) Shell Rotella T 15W/40 in my Ford 6.9L NA'87 van and '92-era 6.5L TD.

That is what I use in the 300K 1979 240D Mercedes and the two 350's in the cruiser.
With the old original 327's I called Chevy tech support, remember when you could do that, and asked if they recommended the Valvoline 5W30 for them like they did the new engines. Answer was, they would be just fine but the tolerances of the old engine would probably drive me crazy adding oil, keep using the 30W.

Good point on tolerances. Nothing more annoying than an engine that requires constant monitoring of the oil level.

Tim has brought up another interesting point: Older engines had different tolerances (which is why most specifed a "break-in" period). Breakin is specified less loudly recently, but IMHO, is still useful.

Also note that "zero consumption" of motor oil is a new concept. In fact, I was concerned that my AMG engine used little oil while being broken in (of course, I changed at 400, 1100, 2000, and 3200 miles at first), and it's settled nicely into a routine of 1/2 qt./4,000 miles now with 30K on the clock.