Generally, the "synthetic oil leaks" objection is a semi-myth. If the engine is old and cruddy inside and the crud is essentially acting as oil seals, then when the inherently more detergent synthetic is introduced, the crud is eventually cleaned out and leaks are revealed. It isn't the oil, it's just the seals and gaskets.
Synthetics also have a greater affinity for metal, ie, they cling better, so any leak looks worse because it spreads out.
I've been using Mobil1, Amsoil or Redline in Fiats and Alfas -and MBs and BMWs -for years, no issues. It's more expensive, but not much, and still a relatively small part of the cost of upkeep on the cars.
Just don't do what a friend's kid did who ran his Alfa OUT of ANY oil - SCREECH! - then asked his dad if he couldn't just add some oil NOW and it'd be ok?
I've been running 15/50 Mobil 1 for about the last 3 oil changes, and still, no leaks. (Yes thats about 9000 miles.) The last time my engine was open was before I was born. Granted it sat for 30 of those years, but that doesn't do anything for seals and gaskets either.
You strike me as fairly "old school" in your view of motor oils. I am, too. I think the Formula Shell 20W-50 would do just as well in your GTV as the Castrol. If you were racing, running a turbocharger or going for extended mileage between oil changes then synthetics might make sense.
Conventional motor oils have improved markedly over the last few years so that the quality gap between them and synthetics has closed significantly. But not the price gap.
Maybe you should be asking yourself a different question: Do I need a 20W-50 oil? If your engine is quite worn, the oil pressure tends to be lower than you wish, and/or your Spica or carbs are leaking fuel into the sump, then maybe you do. Your owner's manual probably specifies a 10W-50 oil. So that's what you (or a PO) should have used 30 years ago. Alfa specified that oil knowing it would shear down to a 40 or even a 30 weight with use and a some fuel in the oil. But that was then.
If your engine is in pretty good condition, you might find you can run a lower viscosity motor oil without harming your engine. In fact, considering that most engine wear occurs while your oil is coming up to operating temperature, using a lower viscosity oil that comes up to temperature more quickly than a heavier oil does can lessen wear.
A personal example: I have a 71 GTV with Spica. The PO had 10W-30 in the car when I bought it. I had the oil analyzed and found it was acceptable from the standpoint of wear metals and it had no measurable fuel contamination in the oil. However, I was not happy with the oil pressure once the engine was warmed up. I replaced the oil with Chevron Delo 15W-40, a robust conventional heavy duty oil. No problem with the oil pressure now, although the oil takes 10 or 15 miles of driving to get up to operating temperature. I have bought a case of Formula Shell 10W-40 to try out next change. It should provide good oil pressure if it stays in grade and it should warm up more quickly than the 15W-40. We shall see. The point is that you should experiment to find out what's best for your individual car.
I personally would not use Delo 15-40 in the Alfa. It is as marco246 says, "heavy duty". I DO use it in the boat, but that is twin diesel Volvo TamD63 s which are each 370HP turbos. Great for boats but I would say too heavy for an Alfa. Just my opnion. I do use GTX but may give Shell a try.
One of the best Alfa mechanics around recommends Shell over Castrol GTX; his other pick is AGIP, all in 20W50 grade (for warm weather driving) or 10W40for colder weather. When he opens an engine that has been maintained a long time with Castrol GTX, he finds more sludge than he likes.
His faith in Shell products goes back to his training in Maranello - in the days of the 250LM.
I always used Castrol GTX 20W50 myself, but based on the information in this thread: http://www.alfabb.com/bb/forums/giulietta-giulia-1954-65/46388-changes-oil-formulations-classic-cars.html, I decided to switch to Shell Rotella 15W50 for my Spider two weeks ago, because this oil is supposed to have better Zinc content.
I was slightly worried about lower oil pressure, but even though it was still in the lower 90's when I took it out for a long drive, the oil pressure was actually better, if I can believe my gauge. So far, I'm happy, and I wanted to share this feedback with y'all. It cost me like 10 or 11 bucks a gallon, so although maybe slightly cheaper than Castrol, still not at the Shell price that LM mentions....not sure about the zinc content in that one though
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