Yeah man, nobody's ever heard of diesel engine dying from excessive blowby. Unless it's got other symptoms yer 310k mile 300D oughta see 410+ one day with regular oil changes. Dont be duped by "high mileage syndrome" that claims ALL engines are near death at/near merely 299k miles.

Meanwhile if you like weird mystical blowby tests, try the equally scientific "poorman's compression test" based on cranking the engine without glowing the plugs - say after engine's been warmed enough to barely move the temp guage needle like 1/100th inch where if it fires up immediately with zero glow then it's got decent compression.