This an oil filter you speak of? If so, then yes, they've been ruled suppressors. So are empty soda bottles and other disposable DIY methods that are intended to reduce the report of the shot. Some large muzzle brakes that are take apart and have just one baffle inside of the brake are sometimes considered suppressors, despite they aren't even modulators let alone suppressors.
If you can't have cans in your state and really want 'em, then move. It's really the only choice unless a stroke of luck occurs and you have your legislature legalize them and SBRs like here in WA. If it comes to the tax stamp and the cost of the can, then maybe suppressors aren't for you. They aren't for everyone. New laws make individual purchase just as easy as a trust now, but a trust will give you much more freedom and protection and you can have your wife as a co-trustee and have beneficiaries too; you can even loan your cans to these people (though I don't do that). A trust from a good, knowledgeable NFA lawyer is paramount. My NFA lawyer that put my trust together for $500 (another $500 will get me a full estate plan) has perks others don't and is easy and done by a pro. He's also a veteran and NFA collector, so that helps.
After the trust, I got my first can and paid my taxes. Since bought several cans for several rifles and I still use my trust, no way I wouldn't. My wife has her own stuff, and we can use each other's stuff and each of us can buy or sell as they see fit (though we don't sell weapons, only buy keepers). They are also dedicated cans for each rifle and I don't have junk; my cheapest and loudest are my AAC 556SD's (basically a shorter QD version of the M42000 with a less effective baffle stack). But they work on my M4's. Don't really own any multi-can cans nor do I use them that way. All are dedicated. It takes time to do this and money, but persistence pays off.
Bottom line is just to cover your ass. Please. Contact an NFA lawyer in your area, ask them these questions, it's free to get advice usually, and you should consider a trust if you can. Not a DIY, one done by an NFA lawyer, and make sure to include your wife or whoever as a trustee also. If single, you can always add a spouse later and get it now for $250 (from my lawyer). Remember, ATF will okay a can on a faulty trust and if it comes into question, it's your ass, not theirs for okaying it. And each one you got that way will be an additional count. Please cover your ass.