I have 90 weight straight gear oil in it now and i feels like it needs just a little more stiffing up would 85-140 weight sythetic gear oil stiffin it up a little more if not what will?
Gear oil is rated different than motor oil. The higher viscosity, the "stiffer" it is going to be. You should be looking at viscosity at basically low temperatures. A common spec is going to rate the viscosity @ 40 degrees Celsius. This is going to be close to 100 degrees Fahrenheit. Any multi weight oil (10w30, 20w50 85-140) will have a viscosity near the low number the same as straight weights. There are ranges they fall in, but here is an approximate viscosity @ 40c .
Synthetics will naturally have a less viscosity at lower temperatures than fossil oils.
The higher number (ie the 140 in 85W140) isn’t going to do you any good in a damper. It won’t see 200-300 degrees where the high number starts to matter.
Here are some more places to look at some data.
Find some 60 or 70 dino oil @ an independent motorcycle dealer that caters to the old HD Shovel Heads. It will be some of the heaviest oil you will find.
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