I have read lots on this but wanted some opinions here. Supposidely my car was always changed with synthetic, according to the shady used car dealer. Of course I was never told when the last time it was done and there is no sticker so I assume the previous owner always did it himself. Its been almost 4000 miles since I got the car, and I have no time to do it myself, and I am not paying $80-$100 for jiffy lube to put synthetic in it for me. SO tommorrow I will probably just have jiffy lube put the cheapest stuff in possible. Is this big deal, and what viscosity should I go with? 10w-40? Its a 2000 Mustang GT with like 97k miles. I also dont plan on having it much longer since I am sick of the 5 speed so I am not worried about extended motor life like 80000 miles from now. I am looking for a Lincoln LS already and I just got the stang 8 weeks ago.