most ppl go with 20 weight fork oil or under for the street and maybe a little higher for the track. the thicker the oil, the sooner the damper will start to leak tho. the properties of fork oil are heat and cold resistant unlike engine oils which change viscosities as temps change.
ur in an area that sees drastic climate change temps thru out the yr so fork oil I think wud be best but if u used engine oil I don't think it wud make much of a difference to b honest I mean the damper only moves an inch or two at a time and that motion is not a continuous motion so viscosity changes aren't really gonna be an issue