TOPIC:

I have a 1983 kz750n Spectre. I need to replace the fork oil and I was wondering if there would be negative effects if I use the engine oil I’m running. It’s the shell rotella t4 15w 30

Anyone have an idea if that’s okay?

It is NOT ok. BUY FORK OIL! Fork oil is not the same as motor oil, salad oil, baby oil, etc. ! BelRay makes excellent fork oil, but I'm sure there are other good brands too. Ed

It will work but will turn to snot if not changed regularly. Don't ask me how I know.
Steve

What weight? Because my manual calls for 5w-20

If the factory service manual calls for 5w-20 fork oil, then why not use that?

The factory manual for the KZ750E and H calls for SAE 10W oil. Motor oil would work fine but I have no idea where such coil could be found. 15W-30 is thicker and the fork would not respond to small bumps well and the ride quality would be more rough than should be the case. Simply installing fork oil is the obvious solution.

BTW, 15W-30 engine oil should not be used for the engine either. That's car oil and not only is it thinner than recommended for the engine, it can also cause your clutch to slip. You would be better off with some 15W-40 diesel engine oil which works very well in motorcycle engines.

Edit: some posts above showed up while I was typing. Regarding 5W-20 oil is concerned, good luck finding some. I'd use 10W fork oil and be done with it.

Yes, Ed is right about the multi-viscosity fork oil being extinct. You should also be aware that the viscosity numbers on fork oil have more to do with marketing than actual viscosity, so you may have to experiment some to find what your forks like best. Somewhere I have a chart that shows real world numbers from actual oil testing. I'll see if I can find it tomorrow and post it.
Motor oil is not really a good choice. Forks never get as hot and they need serious anti-foaming agents.

Here is the chart I ganked off the interweb. The first version is as I found it, the lower one I rearranged and color coded according to centiStokes.

TexasKZ wrote: Here is the chart I ganked off the interweb. The first version is as I found it, the lower one I rearranged and color coded according to centiStokes.

What the hell is all that? Looks like my IRA and 401? I can't ever figure that out either. All I know is my broker takes 1.5% a year.

On the cheap? NON-DETERGENT motor oil! Walmart sells it! 20 wt. $2.99 a quart. That's if you can find it locally. Old engines love it. ND does not foam up.

It is a table that shows the actual viscosity of the various oils at 40 degrees and 100 degrees C. It also shows the viscosity loss ratio and a viscosity temperature index.
It clearly shows that the advertised weight has no relationship to reality, and that the numbers vary widely between brands, and even between different products from the same brand.
I believe the chart would be very helpful to someone wanting something different from what they have been using.
If it is too confusing, then just use whatever you like and be happy.

A substitute for fork oil would be automatic transmission oil. but please just go buy fork oil and am guessing 10w would be a good choice with stock springs and components. Also Shell Rotella T4 10w40 diesel oil is an excellent alternative to motorcycle specific oils. Shell Rotella T6 is a full synthetic diesel oil 5w40 which is also widely used in all newer motorcycles. My 2 cents