saito engines are a little more expensive than OS 4stroke engines.
saito's are lighter weight, a advantage unless you are looking for nose weight.
saitos recomend higher nitro content =more expensive fuel cost here is Horizons over view
http://www.horizonhobby.com/Products...ProdID=SAIE100
every one at my club who owns one thinks there great and has had little or no trouble using them.
that being said all my 4stroke engines are OS.
there reliable, powerfull enough, easy to tune, will run on 5-15% nitro fuel like all there other engines.
there a little heavy compared to saito's but it hasn't been a issue, more like a advantage in some cases where nose weight is needed, often warbirds need nose weight to balance out anyway.
my Hangar-9 P-40 is powered by a OS91 surpass and flys fine, what brand of P-40 are you working on