This is the primary reason I don't like Morgan fuels. I can't find any information on how much oil they formulate. Are you sure the 15% you quote is oil content or nitro content?
If it is oil content then I'd say no, 15% is not enough for a 4 stroke.

What does your engine manufacturer recommend? I just acquired a Saito .62 and, while I don't have the manual in front of me, I believe they recommended 20% oil content. After it is well broken in you "might" get by with an 18% fuel content but ONLY if you run it carefully.

I've flown control line for years where they use a lot of the older iron piston/steel cylinder and plain bearing supported crankshaft technology. In an engine such as the venerable Fox 35 Stunt, an all castor fuel with MINIMUM of 28% oil is required. Even with the more modern ABC and ABN plain bearing engines a minimum of 22% with a 50/50 castor/synthetic blend is recommended for maximum life.

I have a 2 stroke Thunder Tiger 46 Pro in my trainer. It features ABN metallurgy and ball bearings. I am running Wildcat 2+4 stroke fuel with 18% all synthetic but I needle it a bit rich. No synthetic can protect against a lean run better than castor oils. At the end of the day we also run all the fuel out of the crankcase and prime with an all castor prime bottle to get some protection for the bearings.

This link explains why Wildcat says a 4-stroke engine needs MORE oil than 2-strokes:

http://www.wildcatfuels.com/container.html

FOUR STROKE FUEL FACTS
There are many myths about four strokes and fuel composition; we will simply state the facts.


First - Four stroke engines require more lubrication than do 2-stroke engines. This is do to the number of moving parts. Unlike a 2-stroke, a 4-stroke does not have the benefit of raw fuel and oil pumping through its crankcase for lubrication. A 4-stroke draws it's fuel in from the top of the cylinder by the down stoke of the piston. Lubrication is accomplished by blow-by at the BDC (bottom dead center) and run-down of oil through the lifter tubes. This oil must migrate to the bearings as well, therefore, volume is essential. (See disassembled 4-stroke -vs- 2-stroke below for comparision). Yes, despite the nonsense that some would try to get you to believe it's that simple.

Second - Wildcat recommends the lubrication package be totally synthetic. The use of synthetic oil is important in four stroke engines to prevent gumming and varnishing of the lifters and coking of the valves. Varnishing will occur on castor based fuels. This can lead to sticking valves, which will cause the valve set to get knocked out of time. Coking of the valves will cause improper seating of the valves causing a reduction in compression and incomplete combustion. Four stroke engines also have high exhaust gas temperatures and the use of synthetics greatly reduces carboning at the exhaust ports.

Third - Some fuel manufacturers claim that the number of moving parts in a 4-stroke is irrelevant to the oil content of the fuel, and they produce and recommend oil contents lower than that recommended by the engine manufacturers. Remember they do not provide the warranties for your engines they have no investment in your engine. Wildcat fuels meet or exceed the engine manufactures warranties of all common 4-stroke engines manufactured today. Because of the higher oil content, WILDCAT 2 & 4-CYCLE fuels can be run in any 2-stroke engine (except ducted fans) where a fuel with total synthetic oil is desired.