Manitou specifies 5W-40 multiviscosity fork oil for their shocks and they seem to be the only manufacturer specifying 5W-40. I can find plenty of bicycle fork oil, but just about all are single viscosity, like 5, 10, 20 weight. Motorex does offer a 5W-40 fork oil, but it's not readily available.
1. is 5W-40 fork oil the same as 5W-40 engine oil?
2. any known and available "bicycle fork oils" compatible with Manitou forks?
3. I'm hoping the LMS (local motorcycle shop) has 5W-40 fork oil, but if not, any recommended viscocity?
too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter
While the oil base may be similar, motor oil may contain chemicals harmful to O-rings. It's important to use an oil specific to the task. You might ask Manitou about motor oil compatibility for their shocks, otherwise stick to one designated as a fork or shock oil.
2004 Litespeed Siena, 1996 Litespeed Obed, 1992 Miele (unknown model), 1982 Meile Uno LS.
I don't see why you'd need multigrade oil in a fork. Multigrade oil is designed to offer certain characteristics at a wide range of temperatures. You'd see that in a car engine of course, but I can't imagine the oil in a fork would change more than a few degrees during use.
I'd get some 5W-30 motor oil and try that. The higher viscosity number is basically meaningless for this application - it's a measure of viscosity at high temperature, which your fork should never reach. If you want to stick with fork oil, try 5 weight. Should be basically the same.
Becareful buying/selling bike parts on-line. I learned the hard way. :(