[101Volts]

First, welcome. I'm riding a 1984 Caprice and a 1990 Mercury so you're not the only one on here with a car of such an age.

Re: Your questions regarding synthetic oil: In early years (This is over 40 years ago by this date) at least some synthetics caused seals to enlarge or shrink. This most likely isn't an issue anymore and I'm running the Caprice on Pennzoil Platinum 5w30, no issues. There's also the part that a synthetic can clean up an abused engine so well that it shows leaks (just like when detergent motor oil hit the market, whenever that was) or it could break sludge loose quickly and starve the engine of oil in some cases of abused engines, but you just had the engine worked on.

Dad's van, a 2000 Caravan, did end up leaking oil once I started using synthetics. However, Dad has/had (I'm telling him about it if he does it) a habit where he'd start the van and, before oil has been pumped, switch into gear and drove. I'm not saying "Warm the van up for minutes", I'm saying let it idle for five to fifteen seconds (depending on temperature and the engine) before switching to gear. Now when the van starts we hear metal on metal contact somewhere until the oil has been pumped and the crankshaft rattles at the drive belt when hot.

As for the temperature/engine oil suggestions, I'd just stick with a synthetic or biosynthetic 5w30 year round if it were my car (and I'm using a semi-synthetic, Castrol Magnatec 5w30, in the Mercury right now.) The recommendation for 10w40 was written in the 1980s or earlier keep in mind, oil wasn't quite to the same quality as it is today. 10w40 now is a bit overkill if you're going for economy (and it might not even be the best thing for engine longevity now either with synthetics available) and the reason being why they recommended 10w40 then was for extra fluid strength for reduced chances of metal to metal contact, but synthetics hold together better (even though they're thinner) and you don't need something as thick with a synthetic. Or that's what I've been told in the articles I read... And keep in mind even though it says "Synthetic" on the bottle, not all are made equal. Even what it says on the bottle doesn't tell you the whole story, Mobil 1 0w30 is literally thicker than Pennzoil Platinum 5w30 both at start and operating temperature. Mobil 1 0w30 is however thinner at startup than the same company's 5w30s.