I forgot to mention that our manifolds are hard anodized aluminum.
 

I don't think they are interchangeable.  The O-seal requires a full flat face for the o-ring to seal.  The SAE straight thread o-ring boss does not have enough surface due to the chamfered recess.
If the O-seal o-ring is fully contained in the fitting and the groove ID covers the chamfer in the SAE port, then it could work.  The O-seal also depends on the quality of the spotface surface of the SAE port which is not normally made with a seal-quality finish.
I would not risk it without testing.

Ted

what is an O-seal? like a DIN 3852 adaptor (with the oring grooved into the bottom of the adaptor shoulder)?

I try to check SAE J1926 from net, and Ted is right about the shape of the surface, however it will depend on the mating surface of your assembly. I mean if mating surface is a flat surface, this may solve your question.

please note that when o-rings are used in such assemblies results usually problematic. not to let any problems I would recommend you to take into account:
- Environmental conditions (o-ring material may react with the env.)
- Make sure that o-ring chamfer is appropriate (specs are available for such issues) you may check NASA site and search for it.

Here is the brennan suggested equivalent:
http://www.brennaninc.com/ProductDisplay.asp?item=N6400-04-04-O-SS

to the Swagelok the OP is using:
http://www.swagelok.com/search/product_detail.aspx?part=SS-400-1-4ST

The Brennan is not an equivalent.

The Brennan would only be equivalent to this Swagelok fitting:
http://www.swagelok.com/search/product_detail.aspx?part=SS-400-1-4-OR

Perhaps Brennan should have suggested this as their equivalent to the Swagelok, of course Swagelok would not agree:
http://www.brennaninc.com/series.asp?item=C6400-O

Ted

Ted,
After following up with the Brennan salesman, he stated that the catalog page showing the N6400 fittings is in error.  The fittings are indeed ORB type, and the image (and name)are in error.

I will let you know if this is indeed the case.  (I prefer swagelok, but Brennan was 1/2 the price, and the accountants insisted we try Brennan.  We will see if we pay for that savings down the road with service issues!!)
  

Thanks for the feedback.

Good luck.  I have used some Brennan fittings because of price and they were operationally successful.  Other of their T and cross fittings were brazed assemblies and I was not comfortable using them in place of forged or barstock fittings in certain applications.

Ted