Well, making an extract stout you must be doing a steep. Basically, take the advice I gave on the full grain stout on the adjunct grains...minus oatmeal...and apply it to your steeping grains. Oatmeal will require a partial mash. Can you do that?
It is not really expensive to convert to full grain. After the initial investment you will save money. For less than $100 bucks you can convert. You’ll make your money back in money saved after four brews.
If you are just going for 5 gallons at a time...it will be less than $100. A good igloo cooler, an SS braided hose, and a few other parts is all it takes.
Yes I do a grain steep. I have used Oatmeal in my stouts but it was the type of oatmeal that did not need to be mashed. Do you ever use brown sugar, molasses or maple sugars in your stouts? I have used Golden Naked Oats, Rolled Oats along with Special B (Castle) as steeping grains in past Stouts.
You are getting good advice on those stouts. Even doing a partial mash with some oatmeal will help that head retention and give you some much needed body. The suggestion about malto dextrine is also quite good. It adds body without sweetness and gives your beer more mouthfeel. Add it 15 minutes before flameout.
Bite the bullet and go all grain ASAP. Converting a cooler is cheap and easy. Look into using the stainless steel braid from under sink water lines. It’s cheap and easy. Nearly all home brew stores will pre-mill your gains and the per batch price is undeniable.
Cheers,
knewshound.