<<Question:  Why do we change out the castor oil for the other oils?  Is it
because of the additives and the ashless dispersant and to prevent castor
oil buildup in the motors?>>

Again, from my very rudimentary understanding, castor oil has better Extreme
Pressure (EP) capability that petroleum-based oil and I suspect it is more
"ashless" as well.  2-strokes have very little oil and burn it all and
therefore have a lot of boundary lubrication requirements.  Also, for
break-in you want to allow the mating surfaces to grind off the mountain
tops of the surfaces without melting - the characteristics of an EP lube.
This is especially important for cam and lifter surfaces.  Also, that's why
the need for non-detergent oil during break-in as it is 100% oil and doesn't
have any non-lubricating additives.  Castor oil probably has little or no
detergent or dispersant capabilities, so it can't be run for any extended
time.

Also, "Synthetic" oil consists of a more "pure" molecule, still constructed
from petroleum oil base-stock, which doesn't change viscosity much with
temperature, while non-synthetic (natural?) oil contains a very wide range
of molecular weights.  In order to increase the viscosity index (VI),
reducing the viscosity change with temperature a VI improver can be added
that causes the molecules to link together when hot.  A 20W-50 oil is made
from SAE 20 base stock and is really a SAE 20 oil at low temperature (the W
stands for Winter, not Weight) that behaves like a standard SAE 50 oil when
hot.  However, the bonds created are weak and when the oil is sheared in the
bearings many of these temporary bonds break permanently and the hot
viscosity gradually decreases with hours of use.  A compensating factor is
that many of the very short molecules evaporate and the net result is that
the viscosity tends to remain more or less constant.  However, the VI will
gradually decrease.  A synthetic oil will not exhibit this "break-down", nor
are there very many short molecules to evaporate and hence it will maintain
a constant lubricating characteristic for a longer period of time.  The
synthetic by itself doesn't have very good EP performance, so an EP additive
must come with the package.  Just to make it more complex, the method of
determining viscosity is "low stress," and is pretty much like dripping the
oil through an orifice at low pressure.  When in an engine there isn't much
of that going on and the important measurement is how well it holds the
metal surfaces of the bearings apart while being sheared (imagine the
relative surface velocity of the bearing surfaces and the small clearance
between them) at a high rate.  Oil tends to behave at a lower viscosity
level under those conditions, especially multi-viscosity oil.  In other
words, a straight 50 oil will have a higher effective viscosity hot than a
20W-50 oil.  I was told a long time ago by an oil guy that you get about
half the advertised effect - 20W-50 oil is better described as a 20W-35 oil.
So then what viscosity do you need?  That's not entirely clear and begs
another long discussion.

Gary Casey
ES project