There is a Thread going about Additives, ~► Here, however, I will resume a little my own experience with such chemical additives, on my Subaru "BumbleBeast" and other cars that I've serviced.
Regarding Fuels: I've tried Octane Boosters in different cars, both EFi and Carb, with Different Results; usually the results depends more on the Combustion Chamber's design; some engines does run well with lower octane fuels, and you won't notice a change in Mpg's with Octane Boosters, but some
(Little) Power gain or Faster acceleration in Some engines.
Lower octane Gasolines actually explodes more Brutally on the Combustion Chambers, than Higher Octane Gasolines; so since I advanced the timing on my EA82 further than stock, I use High Octane Gasolines on it, to avoid pinging; and it doesn't need any Octane Booster additive at all.
Regarding "Stop Leak" Formulas, I really dislike them, avoiding especially the sandy ones for radiators and blocks as they do more damage than benefits; however, I've tried successfully, three "Stop Leak" formulas:
► The Q.M.i. stop leak additive for Manual Transmissions, a couple of bottles of such a thing, rejuvenates the leaky Seals as Magic, and the results are Long Lasting.
► The Liqui Moly / Lubro Moly brands "Motor Oil Saver" additive, really Rejuvenates leaky seals in High Mileage Engines, I never used it on my Subaru, but in my Wife's Car and another two cars, and I almost didn't believe the difference it made; such additive is Helpful if you don't have the time and / or the money for a complete engine Reseal; it elongates the inevitable, but helps you to avoid from running out of oil on the engine, at a low cost.
► My Subaru "BumbleBeast" had a Power Steering Rack Leak in year 1999, it had both steering rack boots torn, so the Leak got even Worse due to contamination of the Seals with road debris. We used Automatic Transmission Fluid on the Power Steering system, since the car was New, as the owner's manual said.
I disconnected the Rubber Hose from the Power Steering Pump, also I detached the Metallic pipe joint, and Connected the Rubber hose to the Metallic pipe on the Power Steering pump, in order to Loop the Fluid, preventing Damages to the Pump avoiding it from going Dry; looping the Power Steering Pump, does Not damage it, as there is No Pressure on the Looped lines, because there is no Resistance to the fluid's flow.
I drove the Subie with the power steering pump looped like that, meanwhile I could fix the Leaky Steering Rack; the parking procedure became as hard as to Steer was, under said circumstances; then I had the chance to remove the Steering Rack, and obtain from the Local Subaru Dealer, a seal kit for it... but the Seal Kit they sold to me, was too Big for the Rack I had, and I ended with mismatched parts.
I learnt on the Hard way, during that year 1999, that Subaru made three different bore sized, hydraulic steering Racks for the EA82; being the racks that uses seals with a Blue Stripe, the ones with smaller diameter; the seal kit whose Seals have an Orangish Red stripe, are for the Racks with thicker diameter, and there also are a kit with seals that has Green stripes, for an intermediate bore sized, hydraulic steering Rack.
That was what the Subaru Dealer's salesman showed me at their countertop, and they didn't accepted the seal kit back, as it was opened and oily... So I ended buying an Used Steering Rack, from a local Junk Yard. I don't know which size of bore / colour of stripe it has on its Seals, as I never opened that used steering Rack; however, it came with a slight leak already, leaking some ATF on the driveway.
So, I removed -almost- all the ATF from the System, and poured there, half amount of valvoline's "MaxLife" Power Steering Fluid with seal conditioners & Stop Leak, for high mileage vehicles, instead; and half amount from a small, Lucas Power Steering Stop Leak additive, and a brand new pair of steering boots from a toyota tercel; and "Voilá"! ... ... the Steering hasn't leaked a teaspoon of PSF since more than a decade ago; but to Steer became a li'l bit Harder with PSF, than how it was with ATF.
I've used SeaFoam's products successfully; the Seafoam's "Trans Tune" on the Automatic Transmission of my Wife's car, the "KiaStein" worked Great; also I've used Seafoam Spray to clean the excess amount of Carbon Deposits, Also, I've used Marvel Mistery Oil on the engine oil & Gas. That's it.
Edited by Loyale 2.7 Turbo, Today, 12:58 AM.