This may be a silly question but here goes:

I'm fairly used to 2-stroke engines and am finallly getting into using 4-strokers.
Got my first recently, a Magnum XLS 52 and want to do a good break in.

Manual recommends fuel with 10% nitro and at least 20% castor oil. Best I could find at the
LHS was a 10% nitro with castor/synthetic blend fuel. I bought some castor oil from the LHS
as well and was intending on adding some castor to the fuel. Question I have is how much
castor to add for a gallon of fuel of this type that won't impede engine performance.

Thanks for any opinions!

i believe 2 1/2 oz of caster will bring it up 2%, or thats the formual i used to break my magnum 52 fs in

mj, Global, the distributors of the Magnums have stated that the 20% castor is an old recommendation made when synthetics weren't as good as they are these days, your off the shelf Castor/Synthetic blend will be fine as long as it's a good brand. Some people don't advise using "full" Castor in 4 strokes because they say it can clog your valves (it doesn't burn). I use full synthetic in my Saito's and they run like a swiss watch.

Taken from Global's FAQ forum

Four-Stroke Engines

These recommendations work well for both break-in and normal running after break-in.

.30 Use 15% ~ 20% Nitro Methane Fuel with 18% ~ 20% Castor Oil/Synthetic Oil Blend Lubricant

.52 and Up use 15% ~ 20% Nitro Methane Fuel with 16% ~ 18% Castor Oil/Synthetic Oil Blend Lubricant

RGM762 has it. 128 ounces to the gallon. If the gallon is 18% castor oil to begin with you'll need to add approximately 128*.02 or a hair over 2-1/2 ounces. Course then you have a 130-1/2 oz of fuel so the nitro will be only 9.8%

Thanks for the input all!

As Cold_Reboot said: "fugett-about-it!".

It is not needed and is an old recommendation.

I broke my first Magnum in with a castor mix and all my subsequent ones with Cool Power Synth.

No problems what-so-ever with this.