Well I wouldn't say it's an approved substitute, but I look at this way. If for some reason the tripod bearing were to fail I'll just replace the entire axle, which comes with both new boots installed and ready to go. The axles are not expensive at all, so that would be my next course of action, I, like you, was not going to pay $40 for some grease/lubricant and then have to wait for it to be delivered.

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2002 X5 4.6is
1999 C5 Corvette
1992 Mitsubishi 3000GT VR-4
1994 Kawasaki ZX-9R
2005 Mazda 6 Wagon (Wife's)
1991 Mitsubishi 3000GT VR-4