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Did a quick search but didn't see answers for a couple basic Q's (isn't that what everyone says?)

Any issues topping off my current standard oil w/ synthetic (like Royal purple w/ zinc) since that's what NAPA has in stock? (For a stock '68 GTO 400 CI).

Any issues going entirely synthetic on my next oil change? Weight?

Thx in advance

Khmike
 

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It's no problem if you top off the oil with synthetic. They can be mixed. What oil to run is like talking religion or politics. Everyone has their opinion and they will make sure you know it. Personally I don't run synthetic in an older vehicle as the tolerances are not as tight and if you have a leak the superior flow of the synthetic will find it.

I live in a hot climate so I run conventional 20w50 and sometimes 10w40 in the "winter". Even in my modern cars in our hot climite I run slightly thicker oil. My wifes Mustang calls for 5w30 but I go with a 10w-30. Everyone says how bad going with a slightly higher viscosity is but after getting 320000 out of my Sport Trak and 226000 from my Aerostar in our harsh climite people will have a hard time convincing me.
 

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I just tried Driven HR 10w40 by the recommendation of Butler and talking to a tech at Driven, if you store your car in the winter it has rust inhibitors and supposed to coat bearings and such for dry start up. It does seem to have quieted the valve train down some vs the Rotella 15W40 I was running for the last year. Butler does also recommend the 15w50 weight. If yours is stock and not rebuilt your clearances might be bigger and like the heavier oil.
 

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Cameo Ivory 1967
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Did a quick search but didn't see answers for a couple basic Q's (isn't that what everyone says?)

Any issues topping off my current standard oil w/ synthetic (like Royal purple w/ zinc) since that's what NAPA has in stock? (For a stock '68 GTO 400 CI).

Any issues going entirely synthetic on my next oil change? Weight?

Thx in advance

Khmike
Valvoline makes NAPA's oil, so you should feel confident to use Napa brand.

Im a big syn fan and I have been for many decades. I use Valvoline synthetic blend/ high mileage, on my car. It has seal conditioners and some other goodies for old engines.

You can safely go full syn and it's great for the engine, but you might also develop leaks, because the syn just creeps into places that conventional does not.

If you do go Full-Syn, I would definitely use the high milage Napa or Valvoline, so that you reduce the chance of rear main seal or balancer leaks.
 

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Cameo Ivory 1967
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Any additives needed?
No, but youre really entering "subjective territory" there. According to Valvoline; although their synthetic oil does not contain any zinc, their additive package far exceeds whatever lubrication properties zinc added.

In other words, you only needed zinc with flat tappet cams and conventional oil, but synthetics do the same thing that zinc did, which is to keep these parts lubricated at start up.

When zinc needed to be reduced for catalytic converters, they simply replaced it with synthetic friction modifiers. After all, that is why we like and use synthetic. It coats the parts.

That being said, please don't take my word for it. If you do like the Napa/ Valvoline idea that I listed, then call Valvoline and ask them yourself.
 
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