Hello all! First time poster; medium term lurker. Bit of a long story, but please bear with me. I want to give relevant history for my question to make sense. I have a 1998 Volvo V70 GLT low pressure turbo with about 184k miles on the clock. My family has owned it since new, and now it's mine. I literally grew up with this car, I absolutely love it, and want it to last as long as possible. Engine runs like a top (fingers crossed). ATF changed 12k ago and transmission shifts like new (fingers crossed). Body is in great shape given how much salt it sees, so it's unlikely to die from corrosion anytime soon. Getting to 250k+ would be great. For the first several years, it saw dealer oil changes. I was too young to remember or pay attention to exactly what was done, but from the service schedule I assume 5k intervals with whatever brand 10w-30 dino oil Volvo used during that time period. Later it saw changes at an independent Volvo shop, again dino with twice yearly changes at 4k to 5k. PCV system clogged for the first time at 147k, resulting in a mosquito-killing smoke show and forcing some seepage at seals. System was replaced with no apparent long term issues. At 155k, it developed an intermittent hydraulic lifter noise that slowly got worse, so I took over oil changes. After fighting a losing battle using different high mileage oils, short change intervals, and cleaning additives, the problem was diagnosed as cracked o-rings allowing air to enter the oil at the pickup tube, and others allowing internal pressure bleed off. Offending parts were replaced and lifters instantly went back to normal with the engine seeming no worse off. I continued to use high mileage oils with 3k-ish intervals until last spring when I decided to do a cross country summer road trip from to the west coast and back. Knowing it would see high ambient temps, mountain grades, and possibly a longer than usual drain interval, I chose M1 high mileage 10w-30 which served me well. After almost 6k of mixed highway and city driving, I had it changed with Valvoline Maxlife semi-synthetic 10w-30. Maxlife got dark faster than M1, but did very well on the drive up to Washington State and then back home, also changed at 6k. I likely could have gone longer with so many highway miles and a couple quarts of make up oil each direction, but I wanted to stick close to recommended drain intervals. It has M1 high mileage 5w-30 in it now for winter, but I'm reconsidering my choice going forward. The engine does not appreciably burn oil. Including a leak from an oil cooler line (subsequently fixed), it lost 1qt in 2500 miles during the trip. The only other "leak" I can see is a slight dampness on the turbo compressor housing that has never left any spots on the driveway. In 500 miles since the cooler line fix, the level has not moved on the dipstick at all. It doesn't necessarily need a "thicker" oil to stop excess oil consumption if it doesn't use much to begin with. At the same time, it does have some varnish up top. Here's a look through the fill cap at the cam and lifters on the intake side of #3. [img]https://imgur.com/a/F8TAG[/img] Obviously could use some cleaning up there, so high detergent is way up there on my personal requirements. Engine has a cam cover with built in bearings so a full top end inspection and manual cleaning is not an easy option. I hope/assume other areas of the top end are at least in similar shape. All pressurized oil passages that I can see are essentially spotless, including original oil cooler thermostat and even inside the old leaking cooler lines. Ironic as it is that I drive a fuel-gulping brick on wheels, I'm a stickler for fuel economy. I don't expect anything magical, just the best mpg I can reasonably squeeze out of it without being afraid to put the hammer down when needed. Whatever I chose must perform in a wide range of conditions: cold starts on a ski trip in Vermont to roasting in LA traffic jams; hauling a trailer of garden mulch at low speed to zooming up a desert mountainside with the dummy behind me mad that I'm doing 83 mph instead of 83.5 as I pass a big rig. That pushes me towards synthetic oil, especially if I could get away with changing it once per year (barring any long distance trips), while also fighting varnish and protecting the turbo when I push it hard. At the same time, I know it's old and I understand the seals may not be in awesome shape. I spent a weekend fighting oil cooler lines just so it wouldn't coat the whole underside with oil mist and leave big ugly drip spots every time I park. I don't want it to make a similar (or worse) mess if a synthetic oil finds a weak point to escape. To counter that concern, almost 30k using oils with seal conditioners and high detergents had to have done some good. Varnish is starting to thin where the cams throw oil up top. It used to puff a sizable blue smoke cloud on cold start after sitting because of weak valve stem seals; now barely a wisp comes out, even after a hard highway drive and sitting hours overnight. I read up on Valvoline Synpower with Maxlife Technology, and that looks like it fits my requirements. Full synthetic, standard range viscosity (Resource Conserving rated too), good performance at extreme temps, and with extra detergent and seal conditioners. Pennzoil Platinum high mileage also appears to fit the bill with similar kinds of marketing on their website. Either of those worthwhile to try at the next oil change (sometime in the spring)? Other suggestions you've had good experience with? Problems you foresee with this kind of switch like suddenly leaking like a sieve despite, or because of, the history? Not keen on going back to dino, as it obviously was breaking down and leaving varnish even with sub-5k OCI. TL;DR: 184k mile turbo brick; past 30k on high mileage oil. Short trips, long trips, trailer towing. Looking for full synthetic w/ extra seal conditioners and high detergent.