There's basically no compression when an engine isn't running at least at hundreds of RPMs. It's a non-issue for the gears. You can turn a 4-cylinder crank wi

There's basically no compression when an engine isn't running at least at hundreds of RPMs. It's a non-issue for the gears. You can turn a 4-cylinder crank with your bare hands. There's very little load, you're just overcoming the static friction of bearings, piston rings, and compressing valve springs. At the static to creeping rate we're talking, the cylinder fluids leak right through the piston ring gaps. If you want to worry about something with regard to statically loading everything, it's a lack of lubrication that should worry you. The engine isn't maintaining oil pressure when stopped. So you're leaving the crank bearings loaded in one spot while oil slowly squeezes out. "Frictionless" oil film bearings are made of relatively soft metals, so this isn't exactly good for the engine. Also the gearbox isn't dispersing oil as it does when driving. So the gears are slowly drying up while loaded. I'd be more concerned about the engine bearings though. I never leave my manual vehicles loaded in gear when off. It's pointlessly hard on the bearings. Starting is already harsh enough in that brief moment before oil pressure builds. Why add insult to injury? There are oil pressure accumulators on the market in part for maintaining oil pressure when off for friendlier starting of high compression race engines: https://www.cantonracingproducts.com/accusump https://www.cantonracingproducts.com/accusump