[Schulisco]

On a tight engine which has been operated all the time with synthetic oil - yes, they will work and on them I wouldn't expect additional leaks. The OP asked for experiences in switching from synthetic to conventional oil due to availabiliy issues. There I personally expect additional leaks as Ivan also suspects.

Originally Posted by SLNT930

Porsche Classic oils are synthetic. I doubt they would be offering these if there was such a huge issue switching from conventional in older cars.

No. Not all Porsche oils are synthetic!
Yes, Porsche provides own oils, and they're pretty good oils, as always to be expected. BUT: The 20W50 is a conventional mild alloyed engine oil, the 10W60 is a synthetic engine oil! Porsche recommends the 20W50 for the 911 engines up to 2.7l, from 3.0l on the recommend the 10W60.
https://www.porsche.com/usa/accessoriesandservices/classic/motoroil-faq/
https://www.porsche.com/stories/innovation/what-is-the-best-engine-oil/

When it comes to Porsche Classic Motoroils, there are two base oil types – mineral and fully synthetic. The former is produced from natural crude oil. Before the development of synthetics, all motor oils were mineral-based, which is why they are used as the basis for the Porsche Classic 20W-50 Motoroil – making them perfect for pre-1977 up to 2.7 litres, air-cooled Porsche models. All the other motor oils in the range are made from synthetic base oils. These are refined and distilled and broken down to their basic molecules, removing impurities and then allows them to be assembled exactly as the manufacturer desires in order to suit more modern engines. All Porsche Classic Motoroils are also multi-grade, which means they’re suitable for winter and summer.

...

• Porsche Classic 20W-50 Motoroil for all air-cooled, pre-1977 Porsche models up to 2.7 litres (356, 914 and 911 F and G model)

Whether the engine is warm or cold, this engine oil has great compatibility with materials like the gaskets, which are made of cork, in these vehicles and generates few deposits. Specially formulated for the early air-cooled flat engines

• Porsche Classic 10W-60 Motoroil for air-cooled, 911 models from 3.0 litres upwards (post-1977 up to type 993)

When introduced, these Porsche air-cooled engines were developed to benefit from the technical progress made by developments in engine oils. Today’s fully synthetic oil is the best you can buy in order to help maintain your engine and keep it clean, whatever the temperature.

• Porsche Classic 10W-50 Motoroil for transaxle models (924, 928, 944 and 968)

This high-performance oil is synthetic, developed to reach superior lubricating qualities and to maintain its viscous consistency on cold starts.

• Porsche Classic 5W-50 Motoroil Water-cooled 911 models (type 996) and Boxster (type 986)

Developed for the most recent vehicles to be given a Porsche Classic designation, as well as great lubricating properties in all conditions, it also includes advanced corrosion protection as well as neutralising corrosive combustion products.

I consider following the original specs Porsche did in the manuals:



The older an engine is or the more mileage an engine has, the more I tend to use a conventional all mineral-based only oil. Also it depends what previous owners filled in.
The more modern an engine is or it already an overhaul in the last years the more safe I feel with synthetic oils.