The manual for RockShox Megneg specifically calls out to use Maxima Extra 15w50.
When I look for this stuff, for example on Jenson, it is almost 50 bucks after shipping. https://www.jensonusa.com/Rockshox-Rear-Shock-Air-Can-Lubricant
It looks to be the exact same thing, at a cheaper cost on CycleGear (where I can pick up locally)
Air can lube is similar to lower leg bath oil on a fork, as long as it's close to the same stuff, you'll be fine. As suggested above use slickoleum/slick honey or I usually use float fluid from Fox.
The manual for RockShox Megneg specifically calls out to use Maxima Extra 15w50.
When I look for this stuff, for example on Jenson, it is almost 50 bucks after shipping. https://www.jensonusa.com/Rockshox-Rear-Shock-Air-Can-Lubricant
It looks to be the exact same thing, at a cheaper cost on CycleGear (where I can pick up locally)