[marco246]

Using 20W-50 weight oils in vintage Alfas is an article of faith, even though you'd be hard put to find that grade of oil specified in any Alfa owner's handbook. E.g., the owner's manual for my '71 1750 recommends 20W-40 but also lists 10W-30 and 10W-20W-40 (an obsolete specification), presumably for use in cooler temperatures. My '74 2000 specified 10W-50.

My owner's manual for the '71 specifies an oil pressure of 50-70 psi with a hot engine "running fast." I have found that 10W-30 won't give me that, but 15W-40 will. If your engine is quite worn, you may find you need a 20W-50 to provide adequate oil pressure.

Modern oils do a better job of not shearing down than they used to. E.g., the 10W-50 oils specified in the mid-70s were conventional rather than synthetic, and they wouldn't be in use very long before shearing down to a 40 or even a 30 weight. Smart owners (of which I was not one) knew this and used a more robust oil--20W-50--than specified.

So what's the point of this discourse? Just to say that you certainly can use a 20W-50 motor oil without harming your engine, but that--oil pressure permitting--a lower viscosity will do just as well and will provide a slightly better fuel economy, slightly reduced resistance to revving to higher rpms, and a slightly faster warm-up of the engine to operating temperature (and thus less engine wear).

One last thing: there is a great deal of hand-wringing about falling levels of ZDDP (a zinc and phosphorus compound) in motor oils and how this is not good for our older engines. I don't know who or what to believe. But if you use heavy duty engine oils (mixed fleet oils for gas and diesel vehicles) you can finesse the whole issue until the truth emerges. These oils are generally available in grades 10W-30, 5W-40, and 15W-40 and they have plenty of ZDDP. Good brands are Shell Rotella T and Chevron Delo, and there are others.