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Can synthetic fluids be used in Packards?

I always use 100% (not blended) synthetic in all my old cars including Packards. I have never had a problem.

I don’t know for sure that I trust any synthetic ... good old oil still stays liquid form under intense heat , I just gotta think synthetic can’t do that . Just my opinion

I used synthetic (Mobil 1) once in my 56 Senior 374 V8 and upon start up it smoked badly as synthetic oil must have been getting past the rings.

Drained the oil and switched back to conventional (Castrol GTX) and cleared up almost immediately.

I think something more constructive is needed than slap downs. Below is the general defintion of synthetic pertoleum from the junknet:
"Synthetic oil is a lubricant consisting of chemical compounds that are artificially made. Synthetic lubricants can be manufactured using chemically modified petroleum components rather than whole crude oil, but can also be synthesized from other raw materials. The base material, however, is still overwhelmingly crude oil that is distilled and then modified physically and chemically. The actual synthesis process and composition of additives is generally a commercial trade secret and will vary among producers."
Are synthetic oils of great benefit in normal use? They are claimed better in extreme cold and extreme hot, and supposedly less hygroscopic. That is, they absorb less moisture when sitting. The extreme heat property is beneficial under boundary conditions. That is, near metal to metal contact which generates extreme heat.
How about all those claims of less frequent oil change intervals? I don't know any petroleum engineers to ask and can't comment except to say: who knows if they simply have more detergents and such? I did know a MOBIL oil corporate guy who put synthetics in his Packards. Here is what the product sheet says:

"If your vehicle is covered by a warranty, follow the vehicle's oil life sensor or the oil change interval
recommended in your owner's manual. Proper maintenance practices, including frequently checking the
oil level to ensure that the appropriate amount of oil is present, are required to ensure effective performance"
And what do they say about the product itself?
"(Brand name) Synthetic oils are made with a proprietary blend of high performance synthetic basestocks."
Basestocks are from crude oils.

One thing about old car motors is they run dirty, and many use large capacity sumps. Using synthetics is, thus, probably of no great advantage because dirty motors exhaust the additive packages more rapidly than the base oil wears out. That is my case for using convetional oil in old car motors.
Here is a case history I found that is very interesting. I don't have any means to test its validity, but it does support my preference for lighter grades of oil.
"My daily driver had nothing used in it except Mobiloil #30 for the first 99,000 miles of its life. When I started driving it (1959)it was burning a quart of oil every 500 miles. The shop foreman at the GM dealership where I worked said to use lighter weight oil as it would scrape off the cylinder walls easier. I switched to 20 wt and after three oil changes the consumption was down to a quart in one thaousand miles. When I overhauled my engine at 200,00 odd miles I started out with the best rated "DG" oil # 5 weight. I used this weight oil for over 100,000 miles and then went to #10 wt that I still use. I have 18 lbs pressure at an idle and 30 lbs when driving. The pressure stays like this all day driving at 40 to 55 mph. I had the pan off around 450,000 miles because one of my new pistons developed slap. Everything was clean and the bearings showed no wear ( I plastiguaged all nine of them). I would use nothing but the best detergent oil at the lightest weight possible."

As well, the Packard manual recommended heavier grades only to curb excessive oil consumption due to engine wear.

Synthetic oils were discovered by accident in a lab attempting distill heavier basestocks into gasoline.

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I will write here what I understand about synthetics. Oils are a thing many people have many different opinions about.

The main benefits of synthetics is they have a tighter control of the distribution of hydrocarbon chain length. All engine oils have a range of hydrocarbon chain lengths, longer molecules provide better lubrication at high temperatures but can lead to sludge. Shorter molecules are better at low temperatures, but can evaporate at high temperatures. As engine oils wear, some chains break, and some chains re-combine to make longer chains. If a chain gets too long, it essentially creates sludge. If a chain gets too short, it creates a light molecule that evaporates at high temperatures.

Synthetic oils, because they have a better control of hydrocarbon chain lengths, tend to last longer before an oil change is necessary, as it takes longer before a significant proportion of the chain lengths go out-of-range for proper lubrication. In certain applications because of their tighter control of viscosity they are better for wear as well, but generally in engines which have very tight tolerances.

In older engines which were not designed with synthetics in mind, the performance advantages of synthetics are mainly that they allow you to postpone engine oil changes, hence the engine manufacturers which say you can change the oil every 25,000 miles, etc. In an older car which is not driven very much, you probably will want to change the oil more often than the accumulated heat cycles would otherwise warrant it, which probably makes the use of synthetics kind of going overboard. But having a Packard is going overboard anyhow.

A few more comments to the post ,
A good farmer friend of mine has a 120hp international tractor that was bought new in 1972 always used conventional 15-40 oil , never even had the head off of it . Tractor shows over 14,000 hours on the meter.
My wife’s grandfather had a service station in the 50s and 60s in western North Carolina . He told me a conversation he had with the traveling salesman selling STP oil treatment . Basically he took the salesman and poured regular oil and STP in a pan , then took a torch and heated the two ..... the oil just basically bubbled and smoked turned black . The STP turned to a stringy goo that resembled taffy .... now evidently the salesman never stopped again . Correct me if I’m wrong but wasn’t STP synthetic ? Teflon?
Just like then and the same as today’s engines there is substantial amounts of heat from burning extremely clean and near zero emissions .

STP was a solution of elastomer(rubber-like material) in oil - it essentially replaced the worn viscosity modifiers and increased the viscosity. It was made under contract for Andy Granatelli by a major oil company.

Many opinions on here but can I get a basic overall answer on what to run in my stock 55? Valve tick is pretty bad.

I have been using Shell Rotella 15W-40 in both my V-8 Packards. No p[roblem.You may need to replace the lifters.