So, I want to change my oil every 10k instead of 5k. If I buy my own oil, and filter, and do the changes myself, what is to stop me from going 10k per change? Of course at 5k intervals I'd purchase synthetic oil and a filter, for my "receipts" and then keep both for once the car falls outside of the warranty period, or use the oil in my wife's Subaru Outback. What am I missing, and why wouldn't this fool Toyota into thinking the changes have been made regularly at 5k. Thanks.