Be watchful. Always look for the angle-- Ask yourself, "What's in it for him?" "What do I stand to lose?" Stop and think about these things-- Asperger's means it probably doesn't come natural, but it doesn't make it impossible. Just something you have to make yourself think about.
One of my friends is a sociopath (at least, I'm pretty sure). I've known the man for 12 years, and he has never tried to harm me-- he is a well-behaved sociopath. He is an opportunistic bastard-- he will happily talk you out of your beer, your pizza, whatever's in you wallet, et cetera-- but he will not set himself out to harm, and he will (very respectfully) take "No" for an answer when politely called on it.
I wouldn't trust him with a brokenhearted girlfriend (even if he is married-- he would, I believe, see it as simply nature taking its course when they 'somehow' ended up in bed together).
I would, however, trust his advice about a problem he doesn't have a personal stake in. I would just about trust him with my life (literally), and that is not a statement I make about very many people (including some other friends with an immense capacity for empathy).
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"Alas, our dried voices when we whisper together are quiet and meaningless, as wind in dry grass, or rats' feet over broken glass in our dry cellar." --TS Eliot, "The Hollow Men"