The risk is that the oil bypasses your filter if the drain-back valve is broken. The oil will accumulate carbon and particles and cause excessive wear. I'd guess the crank bearings would be damaged first.

Might be a good idea to do very frequent oil changes (like 1000 miles) until you get it fixed.

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles