Depending on the brand, certain types of 3-in-1 oil is close to the viscosity of Rocket Fuel (within normal working temperatures) and for years that's what most folks used for their reels because it's what everyone had in their sheds.
Some very big casters in America/Canada use Penn reel oil (the stuff you get with any Penn multiplier), Rem oil (Remington made) and 3-in-1.
Rocket Fuel is overrated in my opinion, purely because folks tend to recommend it based on the increased speed it can add to a reel but unless you're chasing a speed-demon reel and very long range casting this is of no benefit when fishing.
I'm not saying Rocket Fuel is bad, before anyone jumps all over me - quite simply, most anglers just don't need the extra performance it offers.
I use the tube of oil that comes with any new multiplier I buy on every other multiplier I fish with until it's gone (usually take a very, very, very long time to work through it).
3-in-1 works just fine.
It may not be everyone's cup of favourite beverage but if you already have some you can safely use it. I have a couple of Abu 5500 reels that I use for lure fishing that run wickedly fast and all I've done is clean them thoroughly, dry them thoroughly (arguably the most important step) leave one brake block in and lubricate them with 3-in-1.
I'm not meaning any of the above to be contentious; 3-in-1 can and does work.
Other lubricants can make the reel run faster, but that may not be what you want (it's not what I want from my reels).
If that's your goal feel free to spend a few pennies, flush out the bearings and re-oil with Rocket Fuel.
For the record my out-of-the-box reels can cast a very long way by dropping to one brake block and leaving the lube (which is often a little excessive on a new reel) exactly as it is.
Think about it.