I wouldn't leave my oil in for 400,000kms, but then I wouldn't change it at 10,000kms either.
I'm no mechanic but 50,000kms sounds like a nice round figure to me. Obviously it would
have a lot to do with what you use your car for (drags etc.).
I imported my car November 2002. I put Redline Shockproof Light in my car Feb 2003, because I had crunching when cold and crunching 1-2 & 3-4 with spirited (not stupid) shifts.
After about 500kms with Redline my gearbox improved. When cold there was the slightest notch from 1-2, When hot it would only crunch 1-2 with moderate to hard shift. I ran this oil for 25,000kms with no other probs.
In August 2004 I had to replace clutch so I decided to give Redline Shockproof Heavy a go.
I have done about 1000kms with it. If I am not careful it crunches from 1-2 when cold. When hot it doesn't matter how hard I change it will not crunch.
http://www.redlineoil.com.au/reseller-locations.asp
I would not pay over $120 for Shockproof, check link and ring around