I believe I read it here somewhere many years ago and I just regurgitated the information
I just went and did a couple of searches on the "fish oil" fork question.......and who knows is any of this is true
My 1st 750 fork change smelled like fish oil
"Honda used to ship bikes with a very light non corrosive oil in the forks (ATF and fork oil is hydrophillic like brake fluid - meaning it pulls moisture from the surrounding air). This was a fairly clear oil and was referred to as "fish oil" by the techs and eventually the hobbyists (because it smelled like fish). Part of the new bike prep was to drain this "fish oil" and replace with real fork oil or at least ATF. There have been tales of techs prepping bikes for sale and not draining the fish oil until the fork seals went. I'm sure there are a couple of bikes that went their entire service lives with super soft front ends. A couple of years ago I bought a set of forks from a guy that used to be a tech at mineola Honda (one of the largest Honda bike dealers BTW). He said they were used but low mileage and he had saved them as a backup for his own 75 CB750F. I cracked them open to change the fluid and sure enough - really really bad (I mean foul) smelling "fish oil"."
"The Japanese motorcycle industry used the thinnest oil they could use
and still get rebound damping.
Maybe it was 5 weight, 10 weight at the maximum.
The motorcycle magazines accused Japanese motorcycle manufacturers of
using "fish oil" in their forks, and the motomavens would recommend
switching to an American brand of fork oil, which helped out the
aftermarket lubricant industry.
Unaware of the orifice flow limitations, American riders would install
15, 20, or even 30 weight oil in their forks and they got a very harsh
ride over sudden sharp-edged bumps.
Some American suspension "experts" started talking about air getting
trapped in the forks and entraining in the fork oil because they
didn't know that fixed orifices were so velocity sensitive,
An article in one of the magazines (Cycle?) during the late 1960's/
early 1970's involved filling up forks with various fork oils and
stroking them on a machine while measuring the damping curves.
Spikes in the damping curves were noted, and oil consistency was
questioned.
The series of tests was performed with a certain laboratory oil and
interested riders demanded to know where they could get that oil,
which was unavailable at a price that riders would be willing to pay.
The motomavens suggested using ATF as a consistently formulated
suspension oil, even though there were definitely differences in the
consistency from one manufacturer to another.
The ATF recommendation got back to Japanese engineers, so they adopted
the
suggestion, just as they adopted many other ideas which came from the
Southern California hot rodding community, where the enthusiast was
often working in the *movie industry* and, if they had a degree, it
was in *cinematography*!
The California hot rodders simply had more practical experience than
the Japanese engineers!
As S'mee said, 10 weight fork oil will probably give you a baseline
feel and what you want to do is go out and ride your typical road and
see if you feel like the motorcycle controllable over undulating bumps
and that the fork doesn't feel harsh over sudden, smaller, sharp-edged
bumps.
If your wrists are getting tired and it feels like you're riding a
jackhammer, the oil weight is too heavy to flow consistently through
the orifices in the damper roads."
