This oil overflow line on the transmission was bothering me, it just didn't look right. So now instead of being a single outer layer it's 3 separate objects.
Upper left is mocked up for demonstration purposes using the current texture, which I made half as opaque to compensate for there being 2 layers of mesh now. So it used to be more opaque but still the uniform thing. Lower right is how it looks now.
<-||->
|-> <-| <-|
|->
Tube cross-section looks like this.
Red - original object, outside mesh facing out. transparent true, less opaque texture, render Priority changed so it draws last (-2)
Black - copy of original object, outside mesh facing in. transparent true, more opaque texture, render Priority normal (0)
Blue - shrunken tube, mesh facing in. transparent false, less opaque texture, render Priority changed so it draws after actually solid objects (-1)
So it actually gets rendered blue first (makes center of tube transparent), then black (makes outer bits of tube opaque and skips the middle since that is closer to the camera), then red (covers both with the outer surface, which can pick up specular highlights and such)
It'd work even better if the fresnel effect was taken into account (translucency gets more opaque at steeper angles) but I couldn't find a shader that does that.