Manual Tranny Fluid Observations
 

1991 Civic Sedan owner here, 182k miles, original and only
owner. The owner's manual for this Honda prescribes either
ordinary 10W-30 or 10W-40 (depending on expected ambient
temperatures) for the manual transmission fluid (MTF), with
a change interval of 2 years/60k miles, whichever comes
first. Discussion here suggests that use of ordinary engine
oil is no longer appropriate, due to changes in engine oil
technology over the years. In addition, using instead OEM
Honda MTF has been said, anecdotally and by one
authoritative written source IIRC, to yield smoother
shifting. Smoother shifting of course implies less wear as
well.

Last week I made the switch to OEM Honda MTF. I do think the
shifting is smoother. Not by a lot, but I think what I
perceive when shifting gears is more than a placebo effect.

As a "BTW" when buying my MTF at the dealer's, I asked the
dealer parts folks and one of the service writers whether I
could increase the change interval. They said yes,
suggesting I go to at least three years.

Interest piqued, I figured more recent Honda Civic owner's
manuals might add more insight. The 2004 and 2005 manuals
give a change interval of 6 years/120k miles(!) for normal
conditions; half these numbers for "severe driving
conditions." Go back to 1998, and the interval listed is 6
years/90k miles for normal; 2 years/30k miles for severe.

The OEM Honda MTF cost me about $12 total for two quarts,
which completely changes out my Civic's MTF. Ordinary 10W-30
or 40, for good or bad, costs around $4 for two quarts right
now.

Per year cost ("normal conditions") of using OEM Honda MTF
and the more recently recommended intervals: Just about
exactly the same.

Manual tranny design has not changed much, if at all, from
1991 to the present. A manual tranny is still mostly a
collection of steel etc. gears and synchronizers etc.
mounted on a mainshaft etc. that rotate in the MTF. Barring
further evidence, ISTM the OEM Honda MTF is now for many
older Civic owners the preferred route to go, both from a
wear-and-tear standpoint and a cost per year standpoint,
assuming the limiting factor is years and not miles for
one's driving.