The oil feed line to the variable camshaft sensor, in my opinion, is the cause. Toyota employed rubber lines initially, which had a propensity to collapse and cause significant damage. The issue was fixed after a recall or TSB to upgrade those lines to metal. The timing cover tends to experience minor leaks in addition to that. That repair may be costly (and frequently ineffective). Most people, like myself, just place a tray to collect the oil. There is never enough to worry about. A fairly reliable 6 speed transmission is used. Just remember to perform the advised drain and fill at 90k miles and then every 30k after that. Other than that, it is remarkably trustworthy.
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