If there is a downloadable version of Toyota TSB #DL00303 then I can't find it.
However, I did find a very active thread about it, circa 2003, here:
www.tundrasolutions.com
That thread centered around this TSB and people claiming this or that oil leaked or didn't leak in their Toyota Tundra.
Someone there did contact Toyota and pressed them for details, and this was the best answer they got:
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According to TSB DL003-03 Toyota Genuine Synthetic Differential Gear Oil (P/N 00289-75140) cannot be used in 2000-2002 Tundra vehicles and 2001-2002 Sequoia vehicles. The rear axle oil seals are not compatible and will leak if synthetic differential gear oils are used.
Toyota does not have a position regarding the use of non-Toyota Genuine Synthetic Differential Gear Oil as we do not require its use. If you do use synthetic gear oil Toyota recommends that you do not switch back and forth between regular and synthetic and that you research its compatability with the maker.
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Toyota did not want people using Toyota Genuine Synthetic Differential Gear Oil in their Tundra and Sequoia vehicles because "The rear axle oil seals are not compatible and will leak if synthetic differential gear oils are used."
Toyota went out of their way to say they are only making this claim in regards to their own synthetic oil, and no-one elses. Which naturally left the peanut gallery with more questions than answers - was it something specific about the base oil that Toyota uses? Was it an additive that caused seal problems?
Along the way some people wrote about different chemistries involved in seals and how that could or does play a role. I note the following comment near the very end of that thread:
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In regard to synthetic oils, Mobil 1 is made from polyethylene glycol. This oil is rarely safe to use because of incompatibility issues with seals. However, Amsoil is made from a Diester, which is a vegetable base and is quite compatible with all your toyota seals. You won't have any issues if you use a Diester based synthetic oil.
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Was that true 20 years ago? Is it still true today? Does the basic chemistry claim in that statement hold water? Is there any relavency with engine oils (a reminder - this was about differential oil).