Most of the Bulldog Formation group uses Marvin's stuff and we like it a lot.
The solenoid or check valve is something you really need to do just for belt and suspenders work. What can happen, believe it or not, when the tank is full (it's vented remember), you can end up with a siphoning action and find a quart or more of oil has been pulled back through the exhaust pipes and out onto the hangar floor. ( I know... I know ....but it just happens...)
Lots of head scratching on that one because we all "think" that if air or oil can't flow through the hose, you can't get a siphon effect.
We all believe that but our hangar floors are proof to the contrary. So, the solenoid is real easy to add, (Normally closed),... wire it to your smoke switch and it's no pain and makes shut off clean and sure.
We have also noticed that this is particularly bad with nose draggers. Personally, I think that is because you can actually fill the tank to a point that is above the nozzle ports. Not so easy to do with a tail wheel plane and they don't seem to have the issue as bad... they still do but not as bad.
Nearly everyone we know that has flown a smoke system with others so that it is obvious when one doesn't shut off clean, now uses some kind of check valve or solenoid. I think Marvin is recommending the check valves today but I have the solenoid and it works great. The first didn't work so well and I found that in the closed position you could still blow air through it. I replaced it a couple of hundred hours ago and no problems since.
Now you may not believe any of this stuff about "siphoning" and inconsistent shut offs and that's fine..... but just saying. I don't have a great explanation but I've watched the 10 smokers in our group dribble and drip on many many occasions over the past three years flying a lot of smoking formation work.
The facts seem clear about why it shouldn't happen .... it's just the darn results that are confusing.
I would put a check valve or solenoid on it just to be sure because it's easy .... and cheap insurance!
My solenoid is on the tank/pump.
Absolutely have a controllable shut off valve (included I think) that you can get to while flying if at all possible. Start out with a 1/2 turn open and adjust until your belly is clean after a flight. In my 7, I have it just behind the passenger seat so I can lean it forward and adjust. I typically fly with it open 3/4 of a turn for normal use and 1 full turn for a lot of smoke .... and some belly oil !
Links to my install. https://kvisit.com/ai7Ee
PS... Smoke oil is way cheaper by the 55 gallon drum