Would it be an absolutely horrible idea to just now start putting synthetic oil in my car if it has 151k miles on it? it's a '93 SI, that smokes pretty bad anyway if I don't put some sort of anti-smoke sludge in it anyway, so I figure-my seals are shot, so why not protect the metal components better and continue to use the anti-smike stuff. I am currently running 10w40 Valvoline MaxLife. Could I switch to Mobil1 synthetic withough screwing up my engine?

 

You already are burning up a precious resource and it is not cheap to switch. So consider doing a minor overhaul of the engine-seals and bearings and THEN use the expensive synthetic. This is not a flame-just consider the benefits.

 

What if you have an engine that is old but runs great (no smoke no clanks no bangs) would it be wise to switch to synthetic?

 

keep in mind that the molecular structure or synthetic oil is much smaller than natural fossil oil. While this is a plus because it can better lubricate the engine, if your piston rings let a little regular oil by between oil changes, they're going to let a helluva lot of synthetic by if you switch. Another thing to be wary of is if the engine is old, and even though your seals are still good at keeping the oil in the engine and keep it from seeping out of nooks and crannies, synthetic may find its way out, making a mess, and making you think your head gasket just bought it. While I know this, I myself am pondering whether I should/can start putting syn in my car. It's a 94, so about 9 years old, but only has 54k miles on it... I need to consult with an expert.