I'm assuming you know that Browning owns Winchester - and that FN Corp is the parent for both..../ I don't know if you can still call Winchester for service or if it all goes to Browning now - but on a used gun I don't know they're going to help you or not anyway. The question on an older forend like that - is do they still have that style of forend stock on hand / if you want to replace it. Even if you replace just the forend / it won't match the stock ...so its usually better to replace both if you want to go down that path.
I've been around a lot of shotguns - and I've never seen a forend fail like that....but I doubt its any oil that might have weakend that spot on the forend.
What I would consider doing - is find a knowledgeable woodworker or stock maker - and let them take a look at it / see if they can tell if its been repaired before or if it may be long term damage. They might also be able to repair it / by inserting a piece of new wood into the forend where it failed ...maybe come up with something close to what is there now - so the repair looks almost seamless - but that will mean a fair amount of labor in refinishing as well - unless you can do some of the work yourself.
You could always go to a stock maker and have the gun restocked / probably for around $ 1,500 for something relatively plain.
There is a big leap in quality from an older Winchester model ...even to a current Browning Citori model in target grade guns ...let alone to a Blaser, Perazzi, Kolar, etc..and maybe a much bigger investment. I don't want to be unkind ...but the Winchester model you're shooting has never been very popular - and its certainly not a higher end gun / not a gun that most of us would ever consider having a full length set of tubes made for.