Viscosity of a Russian Imperial Stout

I didn't see grain crush size mentioned here...

Check out this exBeeriment: http://brulosophy.com/2015/11/23/mind-the-gap-course-vs-fine-crush-exbeeriment-results/

He actually made a stout with this, and stated:

When presented these beers in a quasi-blind triangle, I was consistently able to distinguish them. I perceived the fine crush beer as having a fuller mouthfeel, greater complexity, and subtle alcohol twang in the finish. While my preference leaned toward the fine crush beer, the coarse crush sample certainly wasn’t bad! I experienced it as possessing a stronger roast character that was somewhat sharp and quite pleasant, although it came across much thinner than the fine crush beer and had qualities more consistent with Robust Porter than Russian Imperial Stout.

Oats, long boil, higher mash temp, Cara malts, protein rest, and unconverted starches when applied properly will aid to give that thick body and viscous mouth feel.

The mouthfeel comes from medium-sized proteins and unfermentable sugars (dextrins) usually.

RIS can utilize a lot of unfermented sugar, since the style allows for a lot of hops that can be used to balance the sweetness.

A higher mash temperature would always be my first port of call, push it up to 69-71 C.

After that yes oats can help, but you have to be wary of stuck mashes.

Longer boils will thicken the final product by concentrating sugars, proteins etc...

My recommendation would be to alter only one variable at a time and to start with a higher mash temp, next I would change the malt grist, then I would add oats.

Finally I would lengthen the boil.