I've been using 87 or 89 (depending on availability) in my '04 X-Type with no problems observed. Bought the car with 130 K on it and have added only about 3K, though...would issues surface within the 3K, if anyone knows?

 

cellojag, you would most likely have driveability issues within the first 200 miles if you were going to have issues. As for long term issues (this has been argued with no clear answer), that is still out for judgement. I would recommend to use 89 as much as possible if you are not going to run premium, but it is your car and you do as you see fit.

 

I ran regular for about the first 2 weeks I owned it. Noticed my car started making a rattling sound when I accelerated. Thought at first it was from the sea foam engine treatment I had used and thought nothing more of it, thinking it was just a funny Jaguar "horsepower sound." Well a couple weeks go by and I fill it about 3/4 up with regular unleaded. As I drive away I notice a total loss of power, no get-up-and-go whatsoever. I thought it might be the rain, or the weight of the gas in the tank or a bad batch of gas, but it was just the fact that it was regular. I think it eventually catches up with you. Didn't feel like much of a problem before that. Also noticed my engine making a thud sound a couple times, which turns out is from using low grade gas, it affects the spark timing and the pistons slap the engine. After hearing that sound I immediately researched the problem and tried adding premium gas asap. Car drove better immediately and that rattling sound disappeared and haven't heard that clunk sound anymore. Highly recommend running at least midgrade if not premium. It is not worth running regular, you lose a lot of performance and it's bad for these engines.