Many years ago when I owned Austin Healeys, I did some experimenting with
different oils in the lever shocks of my 1965 3000 (they have levers front
and rear) and achieved slightly more control without too much stiffness by
using SAE 30 non-detergent engine oil. The stock Armstrong oil is
approximately SAE 20, so if you use normal SAE 20W-50 engine oil you will
fairly closely duplicate the original damping characteristics. Remember,
multi-grade oils are produced by adding a viscosity increasing additive to a
base oil, in this case SAE 20. The additive acts to prevent thinning of the
oil at higher temperatures (I think they test it at 100 degrees Celsius),
but as the oil is still basically 20 weight oil, it still flows like 20
weight at lower temperatures - hence a multi-grade oil. As our shocks don't
normally heat up very much, 20W-50 oil acts much like straight 20 weight as
far as the shocks are concerned. In my own tests in the Healey, I tried all
the way from straight SAE20W to SAE 50 weight engine oils and found that the
40 and 50 weight oils were just too stiff for normal driving. Also, because
I had one leaking shock, I added a little power steering sealant to the
shock oil after reading a suggestion in a Healey club newsletter. It worked
fine and stopped the shock from leaking a drop!
As far as synthetic engine oil, I am running Castrol Syntech 5W-50. When
the car is completely warmed up (driven for an hour or more) I get about
28psi at idle and 70psi at 2500 rpm. This is a little lower than I was
getting at idle with GTX 20W-50, but just about the same at speed. I don't
think that this low enough for concern, but I am considering switching to
Mobil One 15W-50 at the next change. I think that this may come a little
closer to the characteristics of the petroleum based oil that was specified
for the car. From everything I have read, high oil pressure is most
important at high speed, where good oil pressure is necessary to overcome
centrifugal force in the crankshaft and other rotating pieces to prevent oil
starvation at the bearings. On the other hand, a "light" oil flows better
during warm up and can quickly get out to the farthest ends of the oil
system, such as the rocker arm shaft to prevent cold start wear.
Regards to all,
Rex Townsend
Bainbridge Island, WA
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