By most accounts, the VG Nano is a little more angular than the Defiant. As to below the Hyroad, the Frantic would not be a good choice as it has the same cover, lower RG core. Perhaps a Tropical Storm, Breeze, or the new Roto lower end  balls would work.

The 'best ball to handle heavy oil' is very subjective depending on your needs.  If you assume VG Nano, Vivid, Defiant, and maybe Marvel are your choices you need to decide what you you want a ball to do in heavy oil and what kind of cover prep. you are willing to do to get to what you want.

IMO the VG Nano and Defiant are your more angular balls and Vivid and Marvel are more rolly.  For me, on longer and heavier patterns I prefer rolly balls with as much traction as I can get - which means I am perfectly comfortable changing the cover to 1000 or even 500 abralon if necessary.  For me I prefer a Marvel at 1000 since it is lower RG than the Vivid and gives me a nice smooth arc in the backends.

If you are expecting to play in a bit and swing the ball it is better to get a heavy oil ball that will retain energy and give you some additional side turn in the last 10 feet you probably want to look at the VG Nano or the Defiant.  If you can find one I'd also suggest an Invasion.  I know that the VG balls were more popular but I like the core reaction from an Invasion when playing in a bit.

The new Roto balls (scream?) are going to be 2.57, .02x. Tropical Storm is 2.6x, .018?

I am really looking forward to the new Roto Shout.  The specs. seem perfect for a broken down tourney shot.  I have always been a fan of box-polished solids and a medium-high RG and lower diff. sounds like the perfect combination for getting through burnt up heads with control through the mids.

Ok, so given your answers here is what I suggest:

Vivid at 1000 or lower will probably will be best Storm / Roto choice for heavy oil given your needs.

As far as a step down from the HyRoad I agree with Cobolt Bomb.  Specs. aside a HyRoad is fairly close to a Frantic.  On the Roto side I would add (just in case this was a fallback as well) that the Rising Star is also very close to a HyRoad and a Frantic.  You should take a serious look at the Tropical Heat hybrid.  It is a much lower RG than you think - at 16# the RG is 2.53 and Diff. of 0.040 (remember this is the same core from the Street Rods a few years back) and reactor hybrid it will be a better step down IMO.

There are two distinct approaches to "handling" heavy oil.  The most common is to look for an aggressive shell in hopes of overpowering the oil and opening up the shot.  The less common, but often more effective, approach is to use a less aggressive shell that allows you to square up and utilize the hold area.  The key to this last approach is finding a ball that will hit hard by getting it into an end over end roll.  Balls like the Frantic can work very well on such a shot. I used mine on a 40 ft Highway to Hell pattern that featured little swing area, and had a very good look squaring up 8-10 board.