So then if you use 2 different weights from the same brand it should work
just fine.

But also take into consideration that a "cold start" in a hot climate is at
a warmer temp than a cold start in a moderate or colder climate.

<http://www.finalube.com/reference_material/SAE_Viscosity_Grades_For_Engine_
Oils.htm>

Don't know if this moves the thread forward, but here goes.
Viscosity is a measurement of what we generally conceive as the
thickness of a fluid.  Pudding has a much higher viscosity than beer.
Jello is a pretty normal fluid when we're taling about viscosity.
When the Jello is just made, it's hot and thin.  After time in the
'fridge, it gets rubbery thick.

Liquid viscosities change because they are affected by temperature and
pressure.  The molecules have more or less energy and pack less or more
tightly as the temperature goes up.  .... That's in liquids which obey
the rules of Thermodynamics Newton put together in the 18th century.

NON-Newtonian fluids DON'T FOLLOW THE RULES (those pesky creatures).
They keep the same viscosities at different temperatures and pressures.
 Ball point pen ink is an example of a pressure-dependent non-Newtonian
liquid.  In the pen, it's REALLY thick. At the moment of shear, under
the pressure of passing over the ball and onto the paper, it's viscosity
collapses and it flows onto the paper where it reqains its earlier
thickness and doesn't run all over everything.

Modern motor lubricating oil is made to keep the same thickness despite
dramatic changes in temperature AND pressure.  Since the Society of
Automotive Engineers is loath to change anything they've been using for
awhile (consider the metric system of measurement) they created a way of
defining motor oil viscosity which encompassed the modern
keep-the-same-thickness-across-a-temperature-range "multi-viscosity oil"

10W-40 or 5W-30 really means the viscosity STAYS THE SAME at different
temperatures and pressures.  You can still find 30W and 40W oils in some
stores.  People used to say,"Thirty weight oil" or "Forty weight oil"
when refering to these fixed viscosity lubricants.

If you expect to operate your vehicle in high temperatures all the time,
a heavier range of oil will protect the engine as it first starts
running, but not break down in the heat.  If you're heading to the polar
cold, thinner oil won't turn to jelly when it's cold, but still protect
the engine when it's at operating temps.  You still need the lubricating
capability, so the ranged viscosity keeps the engine protected across
the temperature and pressure ranges.

I remember (decades ago) people in the Montreal Alfa club liked the oil
additive "MolySlip" (with molybdenum disulphide ISTR?) This worked sort of
like a suspended high temperature graphite grease: slippery "plates" of
molecules, which would settle on bearing surfaces and supposedly provided
lubrication when there wasn't much oil in the journals. I remember using
it in my 2000 GTV a couple of times, too. One of the track racers claimed
it saved his engine once when he put a hole in the oil pan and didn't
notice (until the smoke?). OTOH, others warn against additives that might
gum up the works. Dunno. Anybody use anything like MolySlip these days?

snip

It is NOT a 25% change.  Don't believe there is any percentage
relationship between various oil weight ratings.

Oil viscosity changes with temperature.  For straight weight oil each
weight rating has its own curve visc Vs temp.  With multi-visc oils
the additives change the curve so they thin out with temp much less
than straight weight oil, flatter curve.  For something like 10-40, a
thin base oil that has the same visc as 10 weight at the cold temp
measurement point will thin out to where it is the same visc as 40
weight at the upper measurement point.  (Can't remember what the
temperatures are at the measurement points.)

Mickey