I've been using Power Master 15%. It's part synthetic and castro. Can I use Full Synthetic on my 46 size motor? Like Cool Power 15%?

Yes

if its an abc i wouldnt, if a ringed and its been broken in you could but i still wouldnt.
why would you want to switch? and to what brand?
castor oil is your friend, its a little messy but it helps keep your engines rust free and is a better lube than synthetics (here in the USA)

Actually, it depends on what you mean by "better". The synthetic oils are just as good and probably better at lubrication than castor oil is. Castor stands up to elevated temps better, but if one doesn't run an engine too lean, the synthetics are just fine. And then there is the mess issue. Oops, I don't mean to start a castor/synthetic argument here, sorry.

synthetics arnt not better in the USA (in europe they are just as good as castor) when compared to castor. the reason is most fuels use klotz or Avenger synthetic oils which arnt bad but they are not as good at preventing wear compared to castor, and they boil off at higher temps which will fry your engine if you have a lean run.
castor does have some side affects such as gumming the engine and such but its nothing a little cleaning wont fix.
imo their is no reason not to use castor in small amounts (2-5%), im running all castor oil (16.6%) and not having problems with engines gumming up (i do fly a lot so that cant hurt), the only thing i do have is more oil on the wing, which tells me the oil is doing its job, not being burned in the cylinder aka doing nothing.

castor is also recommended in abc engines due to the fact the entire piston is touching the cylinder walls, if you lean it to much and the synthetics boil off their is a ton of wear on the piston/sleeve. also when first starting the engine castor helpes prevent wear from the "pinch" of the abc because the liner hasnt expanded yet.

Sure...but IMO a few (3 or 4 ) ounces of Castor added to a gallon Cool Power will help protect the steel parts from corrosion, and additional protection at higher engine temperatures, like if it goes lean.

The easiest rule of thumb is to READ YOUR OWNERS MANUAL. The engine companies have spent a fortune figuring out what is best in their product. Personally, I agree that Most of the synthetics are not as good as castor in the USA. But if you really feel scared to use it, you can go with something that is just caster oil and add some Mobil 1 or Amsoil, a couple of extra ounces per gallon should keep you safe.
dan

The next engine I see harmed in any way due to the type of oil being used in one of our manufactured fuels will be the first. If I have an engine tuned so lean that the only thing that will save it is Castor oil then shame on me, I deserve to loose that engine. The Klotz oil I use in my gas engines does contain a small % of Castor just like the glow fuel blends. Some of the engines on the market still call for Castor like the Moki but they are also calling for FAI fuel, that's why you should be reading the instructions like Gunner mentioned. Now that I can't get the Power Master with the 18% blended oil I too will be changing brands but it will be whatever I can get from my LHS for the best price, I'm not going to be over thinking the oil issue, they all work just fine.

If you want to use the Cool Power, go for it. It will not harm your engine in any way. It is a very high quality fuel and I've been using it exclusively for several years in all types of engines and many other use it too with zero issues. If you run the engine too lean and burn it up , that's the problem, not the fuel or oil used. Cool Power is a very high quality fuel that uses a high quality synthetic oil.

There's nothing wrong with adding a few oz. of Castor to Cruel Power
The "All Synthetic" guys get bent out of shape when one suggests adding Castor to an all synth. fuel...why is that? (it's not blasphemy ya know )

Virtually all of the engines that I've seen that were HORRIBLY rusted were run on an all synth. oil fuel. I've owned many hundreds of model engines, and have worked on probably at least a hundred of other peoples engines over the last 5 decades.
Where you fly makes a HUGE difference in whether you have rust...
If you live / fly in an arid climate, the chances of you encountering rust in your engines is considerably lessened. Where I live, it can be very humid for long periods of time...that really makes a difference.

All that being said...run whatever ya want...it's only a hobby...your life doesn't depend on it...have fun.[8D]

I agree, nothing wrong with adding it at all. By all means, everyone should do what they feel is right for them and their equipment. I'm just saying that myself and a lot of others run the cool power without adding anything and have had excellent luck. It all boils down to what you personally think is best for your situation based on your own personal experience.

You might be on to something with the climate and rust thing. I've bought used engines on line from really humid areas of the country and a few of them looked like they were pulled up off of a shipwreck on the inside. You see that on occasion around here but it is generally only on very old engines, engines that got crashed into a pond, mistreated engines, etc. when it's 102 in the summer with 15-20% humidity corrosion generally isn't a factor.

I have never said Castor is something to avoid, that's why I have always liked the Power Master, it's blended and has 2 or 3% added. I have also mentioned humid areas of the country adding to the rust problem. I don't care for humid climates so have always lived in Calif. or Nevada. What I do say is I have never seen an engine harmed running fuel with full synthetic. Lean runs are a mute point with or without Castor. Only problems I have ever had with Castor is gunked up bearings and snot on my plane. I think if I would always run my engines dry at the end of the day I would never have any problems at all. My bad!! If anyone is worried about it then by all means add a little Castor, I have never seen an engine harmed using it. Back when I started flying Castor was the only oil in our fuel, I still have a few nice control line engines in great shape.

I think the OP is talking about a Magnum 46, Magnum specifically recommends Powermaster because it is a blend. I'm pretty sure he read it because in other threads he mentioned mistakes in the instructions.