Re: Woe is AlfaBill AD #799

["C.M. Smith" <cmsmith@domain.elided>]

Say it ain't so! Bearing thump sounds ugly, are you sure? If it's the thin
oil then expect all big ends to rumble, not just one. Hard to believe 5W 30
by itself would allow one bearing to fail, but not the others. My vote is
change back to 15W 50 immediately and try it. I caution against driving
anywhere further with 30 weight oil.

My understanding of SAE grading numbers for multigrades are performance
ratings, not actual viscosity. The first number, the "W" or winter number,
tells you that the oil flows like a single grade 5 weight when cold (how
cold I don't know, but pretty cold I'm sure). The second number is the
performance rating telling you the oil performs (i.e. protects the engine
vitals) like a 30 weight oil at all temperatures, but approaches the
viscosity of a single grade 30 weight "benchmark" oil when hot ( again, I
don't know how hot but pretty dam*ed hot) .

The second number is the critical one for engine protection. Alfa specs a
40 weight synthetic as the only oil, as in 10W 40. A conventional 10W 40
multigrade should be ok but obviously will need changing at least twice as
often. A 50 weight oil will also be ok but may cause a little extra
internal drag due to increased viscosity rating, though this is not
necessarily true for synthetics. The only reason to pay much attention to
the W or winter number is the general rule that the thinner the better for
flow reasons PROVIDED (sorry, but this is really important) the oil also
has the correct principal ( i.e. "hot") rating for your engine (or a higher
number).

 So, in my view a 5W 50 would be fine, a 5W 40 probably fine, a 5W 30
definitely not fine because the protection is just not up to the
performance requirements of the 164 engine, i.e. the second number, which
is the performance number, is too low. Unless you experience cold weather
(and I do mean cold) it is unnecessary to use anything with  a 5W rating,
or less, but it will do no harm so long as the regular number is high
enough. You must use a 40 weight or higher. 15W 40 or 15W 50 is probably
doing no harm unless you run in cold weather, in which case you will
slightly accelerate engine wear until the engine is warm.
Michael Smith
Calgary, Alberta
Canada
91 Alfa 164L, White, original owner 

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