What problem are you trying to cure?   SeaFoam makes several products, their gas additive for storage is good, and their stuff to soak the cylinders in after storage seems to be adequate but not especially effective. Spendy stuff here at OReillys in TX...

I don't think I'd be using any oil additive designed to clean up the sludge in the system, as it might break loose dirt that could ruin your bearings......I'd rather see you pull the pan and pickup screen and clean them really well instead.

I did not use it in the engine oil (CJ-5) for the reasons F Bill described...it might loosen something that would then plug an oil passage.

I did use it in the fuel tank, as the jeep had been sitting for several years. I drained and rinsed the tank and then I used a can each time I added fuel initially, trying to keep it close to the right ratio. The jeep just kept running better and better, but it may have done so without the Sea Foam as well....I was using premium fuel, no ethanol.

Here is a pic of the fuel filter after several tanks were run through it...



I tried it in the fuel in a 97 Landcruiser. No real noticeable difference there.

In the nine years I've spent on farmallcub.com, they continue to love that stuff.  I did try it once, and really didn't see that it made any difference.  But who knows.

It was actually 1oldracer that brought it about aviation fuel (avgas [100LL]) lasting 8 months or more in your tank without going bad.  I just confirmed what he said.  I do always let my tank get nearly empty before my Jeep hibernates for the winter.  I fill it with avgas and drive it home and park it in the garage.  I do drive it about each 4 weeks just to keep it limbered up.  It's always ready to go in the spring with no problems.  I wouldn't do that with car gas. 

Note: 100LL = 100 octane rating low lead.  

On our airfield it is going for $5.85 presently. Get it while you can!!