Instead of a ‘lead substitute’, try Marvel mystery oil. Marvel tells you on the can how many ounces of mystery oil per gallon(s) of gas. I use Marvel in the fuel in my A.C. WD-45 tractor. When the fuel in the tractor is about a month old, I drain and filter it then throw that into my '71 Chevy van. No problems. Last van engine rebuild was done using unleaded valves and valve seats in the head. (L-250 6-cyl. w/ 130K on last rebuild). If you use a lead substitute, you will damage your catalytic converter. You could also try a product like ‘Stab-il’ (I think). That stuff acts to prolong the volatility of gas. Use that in the tractor and don’t worry about any kind of a lead substitute adversely affecting your Subaru. My A.C. happens to be a 1956 model. Those engines were designed to use as low as 75 octane fuel. So fairly fresh fuel with Stab-il should keep the gas fresh enough for the tractor for several months. Check the label on the Stab-il container for correct mix. Using both Stab-il and Marvel Mystery Oil, plus a bottle of ‘dry gas’ in the winter has not at all affected the old A.C. engine.