I won't comment too much on the .049's because I've only ever run 3 different Cox's on the test stand with FAI fuel just to see how they'd run. All gave one flick starts and ran perfectly, certainly good enough for sport flying. The smallest engine I've used was an ETA 29, starting back in about '58, then progressing through Frog 500, K&B Stallion, OS Max-III 35, Enya 35, Merco 35 (where I began competitive stunt flying), Enya 45, ST G51, Stalker 61, OS 40/46VF, Irvine 40RLS, Stas's 42RE and now Enya 61RE. Everything up to the G51 was run with 75/25 all castor and from the Stalker on were run on 80/20 all castor except the Stas's which has an iron piston. I'm not counting the multitude of engines I've only run on the test stand where I always use 75/25 fuel. I've never used or found any need for nitro.

Essentially, any engine with an iron piston should use a minimum of 25% oil, preferably castor, but the best way to find out how much oil an engine needs is to start off with a high oil content and check the head temp (using a thermocouple, not infrared) then reduce the oil content a little and recheck the temp. Keep doing this until the temp starts to rise and go back to the previous oil content. Even this isn't a guarantee though because something like an OS Max-S will run fine (as far as head temp goes) with 20% oil but the rod needs at least 25% if it's to survive. It's good to keep in mind that too much oil has never ruined an engine while too little will kill it .

Larger engines can use less oil because cylinder bore area (where oil is needed) increases at a much slower rate than cylinder swept volume (which is what determines how much oil is drawn in).