Hi, right now in my 1994 Sentra I'm running Supertech 10w40. I had no real particular reason why I ran it, the car was pretty much on its last legs when I got it and it's been brought back to life somewhat now, to the point it's an around town kind of vehicle. I basically only ran the 10W40 as I had another car a couple of years before with a severe front main seal leak, and I bought it as a 5 quart jug to limp it around before I sold it, and I had 3 quarts left in the jug and the car came with a quart of Formula Shell 10W40, and it's a 3.5 quart car. I put it in, probably in... August or September? It seemed to run smooth then, but I noticed kind of randomly a giant gas mileage drop, from about 23-25 city, to 20-21 city, with cold temperatures finally hitting harder. However, this week it was relatively warm, and I got 25mpg again. So I'm thinking it's a case where the 10w40 doesn't heat up sufficiently in winter, especially with a lot of short trip driving. Also the engine has sludge too, and my oil is pretty much black after these 1500 miles, so I'm hoping to change it out for that reason as well, at least the detergents are working. As it is now, I'm really happy about its oil usage, even with a slight valve cover leak it's only used about 1/4 of a quart in about 1500 miles. This is with non high mileage conventional Supertech 10w40. However, the seals aren't really good at all on it, the spark plug seals were rock solid, along with the valve cover gasket, they just broke into solid pieces by bending them with my fingers when I changed them. So I'm hesitant to try synthetic, though I've had other people tell me they had decent results switching to synthetic on old cars like this (my friend told me he had a sludged up Civic where chunks of sludge came out of the oil pan, and he did 5-6 3K OCIs with synthetic and it cleaned a lot of sludge out...) but I'm pretty scared. At Wal-Mart right now, ST conventional and synthetic are within a dollar of each other. I've also read some conventionals are just secretly synthetic now as well. I think now with more fracking, synthetics are dropping in price due to more natural gas being found? I know even some bulk oil in drums for oil change places are synthetic blends now, as well (place where I briefly worked had 5W30 syn blend bulk oil....) I had a Volkswagen prior to this, and in that with similar mileage (but a lot better shape, except for a cooling system problem and being hard to work on in general...) I alternated between 15w40 Mobil Delvac and 5W40 Rotella. I actually ironically got about 1-2mpg more with 15w40 compared to the relatively fresh 5w30 in it (friend gave me the car, pretty OCD about oil changes and related stuff...) in summer in the Volkswagen, maybe some gains in compression? In winter the 5W40 Rotella seemed to run fine as well, no mileage drop, so that's partially why I felt OK running the 10w40 in the Sentra, figuring the heavy oil thing in cold weather was a myth/overstated. My father personally got 450K out of a Ford Freestar without going into the engine, and driving it to the junkyard with Rotella 15w40 and 3-4K OCIs (in an engine spec'd for 5W20.) With the diesel oils my Volkswagen ran super nice and smooth, so I was thinking of running a 5W40 diesel oil again, but I don't know if it would cause a mileage drop in the Sentra still compared to 5W30 (it says 5W30 preferred under the hood on the sticker, but makes allowances for up to 20w50 in summer...) but most of the 5W40 diesel oils are synthetics or synthetic blends. So, what should I run? Full synthetic 5W30 high mileage, 5w30 conventional, (high mileage or normal?) or a 5W40 diesel oil? Thanks very much!