Vehicle's History I have a 1998 Toyota Camry 4 cyl 2.4L with 260k on it. For the past 50k (3 years) that I've owned it, it's been run with conventional Quaker State Oil and Purolator PureONE Oil Filters. I work about 7-8 miles from home, in a fairly dry area in the northern US. Engine has always ran smoothly. Always had a few leaks but more on those below. I throw Seafoam or Techron in the gas tank once every 6 months or so. Due to financial reasons of suddenly not being able to afford a second vehicle, I have a new interest in keeping this car for several more years. Recent Work Done Newer Monroe Quick Struts. Newer Michelin tires on steelies but got some very nice wheel covers on order complete with Toyota badges. Patched up a hole in the exhaust recently and now she's smooth as butter again (whole new OEM exhaust next year). Also, recently, I paid $700 for new gaskets on the crank shaft and oil pump. He did the timing belt too. I did the easier valve cover myself and saw no sludge and a relatively small amount of carbon build-up. Since this will be my only vehicle, I completely removed the back seat cushions and backs and made a sleeping platform where my legs go into the trunk. It's quite spacious and I plan to do some weekend getaway camping. I was driving a much smaller car while this was in the shop and I've come to appreciate my full-size Camry much more than before. It runs great now too. I'd say it runs better than half the vehicles half its age. Current Preventative Maintenance If it ain't broke don't fix it is one thing--"Mystery Motor Oil could dislodge some crudge and get up in the cylinders..." or "Switching to synthetic might hurt rather than help..."--but then again, I want the absolute best for the 260k miles old, still smooth, strong engine. I read about Auto-RX and pretty much everything I can read about the topic. I'm on the Toyota forums as well but I figured I need to be on the OIL forum for some of this! I came up with a plan based on reading way too many forums (pretty much nothing is agreed upon by everyone). So far I've added 5 oz Marvel Mystery Oil (only 15% of what's recommended) in the conventional Quaker State Oil. In the future, I think I'll do this around 3-500 miles before each oil change. I don't want to do too much at once to have a lot of micro carbon particles floating around in there. I put the recommended amount of Marvel Mystery Oil in the gas tank; and instead of regular gas, I put premium in. This is all to decarbonize, as a preventative measure. I used only 15% of the recommended dose of Marvel Mystery Oil in the oil because I didn't want to suddenly dislodge anything large. Future Plan Since I have new seals I will NOT use High Mileage Oil! It's up for debate, but from what I've read it's my belief that the high mileage oils' seal conditioners will unnecessarily expand my brand new rubber gaskets that I just had replaced. Not worth the risk. Also, at next oil change I'm thinking to drop the oil pan, inspect and manually clean it, and change the seal while I'm under there. I do feel I should switch to full sythetic oil. First, however, as to not clean the engine too quickly which could do harm, I'm thinking to follow this plan: 1) 5 oz Marvel Mystery Oil in current oil for the next 300 miles then oil change to... 2) Conventional Quaker State oil, PureOne filter, with ONE QUART of QS full synthetic mixed in for about 2000 miles then add... 3) 10 oz of Marvel Mystery Oil for 300 miles 4) Sythetic Blend (Valvoline or Pennzoil, not Mobil 1), PureOne filter, go 1000 miles then... 5) add 10 oz of Marvel Mystery Oil for 500 miles 6) Full Synthetic, PureOne Synthetic filter, run for 2000 miles or so. 7) Pretty much continue with this regimen, but eventually go 3-4000 miles like I've been doing. I'd eventually phase out the Marvel after running full synthetic. I'd still do SeaFoam or Techron in the gas on occasion. Questions Does Marvel Mystery Oil damage my new gaskets? Is this Auto-RX stuff a bunch of junk? Should this or SeaFoam replace the better-proven MMO? Does regular synthetic "condition" my new seals that definitely don't need conditioning? I don't want them expanding. Should I even switch to synthetic at all? I do want to for better protection. Should I use Purolator's 99.0% "synthetic" filter after going to synthetic oil when the PureOne filter says 99.9%? It is my understanding the oly PureOne is more efficient but the synthetic ones hold twice as much debris for 8000-mile oil change intervals, which definitely won't be me. Mobil 1? Mobil 1 Super? Valvoline? Pennzoil? Amsoil worth it? Anyway, sorry for the long post. I've much made up my plan. I'm hoping it's a good one. I just came here to get some experts' opinions to make sure it's right for my engine. Thanks a bunch!