I just add a bottle of Camshield or an other kind of zinc additive to my regular oil changes.

Diesel oil no longer has as high a zinc content as it used to.  I continue to see this claimed, but the information is almost 20 years out of date.  Diesel oil continued to have higher ZDDP concentration for years after it was curtailed in gasoline oil formulations, but just as the sulphur content of Diesel fuel was curtailed, and light duty Diesels were required to meet emissions; ZDDP in the oil was also reduced. 

Additionally there is no benefit to running oil that heavy in a typical street engine.  You would be fine with 5W30 or 10W30.  The oil standards used when these cars were new are now obsolete, but when converted translate to a 30 weight oil.  If your weather tends to be hotter I'd go 10W30.  If it gets colder use a 5W30.  I've run 5W30 in my 289 for over a decade now with zero issues.  If you have a flat tappet cam I would advise adding some ZDDP containing additive.  I simply use STP.  Cheap and effective. 

Now synthetic oil is a whole different animal.  If you poured a 30W conventional oil and a 30W synthetic oil side by side you'd see a marked difference, especially over a range of temperatures.  Synthetic is always going to appear thinner, particularly when cold.  This has to do with how the manufacturers chooses to grade their oil.  When they say its 5W30 it just means it passed the tests tor those oil weights.  Could it also pass the tests for other weights?  Probably.  Synthetic is really, really good oil.  Point being, is there a marked difference between 5W30 and 15W50 or 5W20 in terms of thermal stability, pressure handling, etc.?  Probably not.  I've seen guys run 2 quarts of 0W20 in crate motor racing classes with a claimer rule.  If you do the math by half track the pan is dry, yet that engine runs like that all season long with complete reliability.  Most of those engines are supposed to run 5W30, etc.  I would never recommend doing anything like that on the street, but such an extreme example points to how largely meaningless oil weight becomes when talking about synthetic. 

There are two cautions I'll give on synthetic:

1.  I've had switching to synthetic on high mileage engines cause oil leaks that weren't there before.  I've never had a definitive explanation as to why that is, but my two best hunches are that either the oil molecules are consistently smaller, or there isn't enough additive to replace what the heavier molecules in conventional oil (like parafin, etc.) contribute to keeping seals pliable.  If the engine has been run trouble free on conventional for years and has some mileage on it I would advise sticking to a conventional oil (5W30 or 10W30) and adding ZDDP if you have a flat tappet cam. 

2. Don't try an break in an engine on synthetic.  Use a dedicated break in oil or conventional oil.  Synthetic is so slick that the rings likely won't seat.  You need some friction to break the engine in properly.  Once its broken in, by all means switch over.

Last edited by TKOPerformance (1/11/2023 5:18 PM)

Motor Trend posted an article on using diesel oil in a gas engine just a few weeks ago. 

https://www.motortrend.com/how-to/diesel-oil-in-gasoline-engine/?wc_mid=4035:24072&wc_rid=4035:2214764&_wcsid=C62F70A573CD4C8E6EB5C7420A94B3AB99F402FF93FF3898

If it was blueprinted, do you have the bearing clearances for rods and mains? That would be the lace to start for oil weight. If they are within normal spec, I would stick with a 10W-30. Do you have a mechanical gauge for hot and cold oil readings? 

The Motortrend article goes with TKO.  

Thanks for all of the great information. There's a lot more to oil than I realized.

Although my 289 was blueprinted, the previous owner that had it done didn't have any specs. I am going to try out the Mobile 1 15W-50 for this oil change and consider 10W-30 for my next oil change based on the oil pressure readings I get in the car.

SBFs have a good oil system.  Typically if the clearances were right when it was built and the engine is healthy they need nothing fancy to live a long time.  Conventional wisdom says that such an engine needs no more than 10psi of oil pressure per 1,000RPM.  So if you've got 60psi at 6,000 RPM its fine.  At idle you should show 20-30psi, that's normal warm idle oil pressure, and you may see more like 70-80psi at max RPM.  Again, perfectly normal.  With 5W30 in it I see hot idle oil pressure of 20-25psi and 60psi at 5,000RPM when I shift.  Good enough.  Standard volume/pressure pump.