This topic has come alive again. Recently there has been a lot of discussions about the ethanol mixed gas. It eats a lot of rubber parts: thus ethanol safe rubber hoses should be used instead of the Coline or other flexible lines, Zeniths have rubber o-rings sealing the jet covers, they aren't lasting more than 1 to 2 years max., ethanol attracts water and also scrubs metal very clean, making for the opportunity for rust in gas tanks and fuel lines, and it evaporates about 20% faster. PCA just released an hour long video about Fuel additives, Oils and Oil Testing, very scary, mostly aimed at the newer cars, but still can be considered for 356s. Here's the link:
https://www.youtube.com/watch?v=Oc4TBJPs8zQ&t=21s
What has been discussed on Rennlist and other internet sites is:
Top Tier gas, should be used, premium is best for our cars, it has been mentioned V Power Shell is good. When the gas evaporates it can leave some deposits and any water, that can build up over time, clogging the carburetor jets, Top Tier gas has better/more additives.
Oil should have ample ZDDP levels, around 1000-1300, and be changed twice a year, some mention of high levels of Calcium but have not found and data on that.
Fuel additives that have been mentioned are: Chevron Techron, comes in regular and complete, both could be as used for two tanks before an oil change. Stabil 360 which is used on a regular basis, and Stabile 360 Marine which is a stronger version, Techron Marine also cis to be used regularly year around, along with Lucas Safeguard Ethanol.
All seem to be pretty much the same, here's Techron's list: Distillates, hydrotreated light at 40-70% weight, Stoddard solvent at 15-40% weight, Solvent naphtha (petroleum), light aromatic at 5-10% weight, 1,2,4-trimethylbenzene at 1-5% weight. Polyetheramine (PEA) (detergent), polyether amines at 20-49% weight. PEA is the stuff that cleans and is very strong stuff.
The video that is done by Charles Navarro, who also want to market his companies products, but great information, it's makes you think about what could be happening.
For my 356, I use good gas, Shell Nitro V Power seems to run very good, also Costco, rumored to be Shell too in our area, oil is Valvoline VR-1 20-50 changed twice a year, I also have been using a little bit of SeaFoam now and then, for years, to dry up the fuel system, but then I do drive it year around and in a mild climate. The newer Porsches get Mobil 1 15-50, Premium gas, and occasionally add some Techron, oil changed once a year, again they are driven year around, but only about 2500 miles year.