Fizz , a lot of after markek springs have the rates etched on one end of the springs. If not marked on the springs it can be calculated to give you a fairly good idea what you have.
With regards to fork oil if the have been resprung I'll personally run a bit more of an air gap from standard(maybe 5mm to 10mm bigger). Easy enough to add a bit more if the forks using all travel too easily. With higher rated spring you can usaully afford to rund a larger air gap from standard.
10 WT oil should be okay especially with winter coming assumming the the bike wasn't respung for a 400Lbs rider
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If the guy was a similar weight to you probably springs are like .85-.9 depending on preference. I'm about a 180Lbs clothed and for road .8 or .85 does me nicely.
Cheers
Ben