Engine oils vs fork oils

[Alf]

Hi, guys:

The FJ1100 Yamaha OEM workshop manual recommend 10-30 engine oil for the front forks.

I´ve been reading that fork oils are simply (and cheap) engine oils with less additives. In fact, my friend Mingo, who works in an industrial vehicles spares centre, last week he assisted to an oil course and he confirmed that the fork oils are engine oils with less additives.

My friend Jose Carlos has been testing 10-40w engine oil in his 3CV forks, with excellent results: consistent damping at high temperatures and clean oil after 12.000 kms, when usually the fork oil is always black and dirty when changing

What do you think, guys?. I´ve just change the shock absorber oil in my Honda CBX 750 and I´ve used 5-50w synthetic oil, because the previous 15-30w fork oil is dirty & degraded with only 6000 kms. More damping force is the 1st impression

I am thinking about to use 375cc 5-50w engine oil in my front forks. ... I know, I know..."use the Randy valves and springs"... but while I´m saving to buy those, what do you think?

Regards
alf

[Arnie]

Fork oils are "Hydraulic Oils"  Usually with high detergent and single viscosity. Some of them have a "seal swell" additive as well.

My opinion is that its best to stay with either specific "fork oils" or Automatic Transmission Fluid.
The ATF's come in 3 common viscosities and are much less expensive than "fork oil".

Chrysler ATF+3 = 10W
Dextron 3/Mercon = 15W
Type F = 20W

Arnie

[soundmindryan]

Chrysler ATF+3 = 10W
Dextron 3/Mercon = 15W
Type F = 20W

What? Really?
With all due respect, I have not seen that anywhere.
Reference, please.

[Alf]

I think all the oils may actuate like hydraulic oils if these are in an hydraulic equipment. In fact, very special and tiny oils actuate even at surgical medical equipment, so my question continues in the air, and more after the experiment of my friend Jose Carlos with his 3CV

And using fork oil in my Honda rear shock could be a bad idea considering the high temperature that the oil has to support in a shock, around 40ºC more than in a fork

[andyb]

I've got either 10w30 or 10w40 in my fork (it's been awhile), and it was an improvement over the sludge I found there originally.  Couldn't say for sure if it's better or worse than fancy $15/pint oil.  At worst, what's going to happen?  If it wears out, loses viscosity quickly, you'll end up with funny colored water that smells bad in your fork, but draining and refilling isn't that big a job. 

[markmartin]

Maybe I'm being talked into a couple of pints of snake oil, but I believe the Viscosity Index is a real thing.  It is discussed in the article in the first attachment of the following post http://www.fjowners.com/index.php?topic=3739.0  BTW, using the other attachments in these documents, you can see what the true viscosity of the oil that you are using really is. 

With the little experience that I have, using a fork oil with a higher VI proved to be a much more consistant at different outside temperatures than a (fork) oil with a low VI. I noticed it on the FJ, but I put it to the test on my dirt bike forks.  Same viscosity oil, but with the low VI oil that I first had in, when riding in cold weather the fork action was stiff as hell, especially when first starting out.  Mollassess in January.
With the high VI index oil I've got in them now, it doesn't really matter if it's 34 degrees or 84 degrees, I don't notice it.  I'm not against motor oil, but I'd like to know what the VI Index is before I put it in.  For the few bucks difference in price on a once a year oil change, I think it's worth it.

Oh well, to each his own, but this snake oil tastes good!

[DB Cooper]

For what it's worth, I've been running ATF3 Chrysler for a little over a year and it seems to be fairly consistant, hot or cold. I'm going to be changing the oil over the next day or two, but, seeing as how I'm a little short for my weight, I'm going to try moving up to Dexron (15).
Kevin.