Gorgeous car! Based on your photos it's an absolute steal for what you paid. Chrome bumper MGBs, especially 60s ones, don't come around that cheap often anymore.
If you haven't already, join MGExperience. There's SO much knowledge there on these vehicles.
As said, these cars are simple, especially the older ones, and it's often kind of amazing/alarming at how well even a poor condition engine will run.
Yours looks to be a 68 or 69 based on the pillow dash and the old style vertical slat grille. These are fairly desireable years-pillow dash aside for 68 the cars got some nice changes from the factory(negative earth and an alternator, rather than positive earth/dynamo, plus have a pre-engaged starter and 4 sync transmission). They have minimal pollution gear(side note-I actually prefer the PCV set-up of these cars to what's on my 1970, and recently retrofitted this style).
For fluids, here's the general advice and/or what I'm running:
1. Engine oil-20W-50 unless you have a good reason otherwise. I've run Castol Edge 5W-50, but honestly the only benefit I really saw was better cold cranking at temperatures I wouldn't really drive the car anyway and it's expensive. Many people will say a high zinc oil due to the flat tappet valve train. I have hedged my bets and always done this(especially since I'm now running a new, higher lift cam) but in reality most -50 oils have enough ZDDP(around 800ppm) to keep these engines happy. I've been running Valvoline VR-1 for years(1200rpm ZDDP, the "ideal" level per some), but Castrol in the last few years has introduced a(sometimes hard to find) "Classic" oil with this same amount of ZDDP. If you want something lighter, 15W-40 diesel oil often isn't a terrible choice.
While I'm at it, I run an FL-1A filter(or equivalent). That's assuming your car has a spin-on adapter with the filter sitting upright. If yours is a 68, you may still have a cannister filter-this is a fine set-up except can be fiddly to not get to leak and the cannister filters aren't the easiest to find. In any case, assuming you're using a spin-on filter, the FL-1A fits fine and you have lots of choices plus tons of filter media. There are some threads on MG Experience about specific filters-some don't have threads formed exactly right and can damage the soft aluminum on the spin-on adapter(I think Wix filters have this issue, or at least use to) and others interally can foul on the standpipe and interfere with flow. I currently have a Baldwin on mine just because it's what I happened to have in the garage when I needed one, but Purolator/Motorcraft are fine and usually what I use.
2. The gearbox gets 20W-50 also. The 4 sync box is tough(aside from the 4th gear baulk ring liking to crack-you'll know this if yout transmission grinds shifting into 4th). It's not stressed a ton and you don't need wear additives. I've usually used Supertech brand at Wal-Mart, but really have just gone with whatever is cheap when I needed to fill.
The non-overdrive transmission isn't too picky. If it's an overdrive transmission, I'd suggest NOT deviating from the recommended oil. It depends on the proper hydraulic pressures to engage the overdrive(in the 400-420psi range). In fact, a first line diagnostic for a non-working OD is often draining and filling with 20W-50.
You should have a "top fill" transmission, which has a dipstick tucked behind the center console. Look for a rubber plug on top of the transmission tunnel. Once you get it out the first time, put a heavy zip tie or piece of wire around the dipstick handle as it will make future removal easier. This whole operation is MUCH easier from the passenger footwell. To fill, you will want a funnel with a long flexible spout, and be ready to still probably make a mess. Fortuantely this is not an operation you will need to do often.
3. GL-5 spec gear oil has issues with "yellow metals" and can cause issues with the thrust washers and other parts in the differential. I forget what brand I've used for GL-4 spec in the past-it was something I bought on Amazon. It's out there-you just have to search for it. The drain and fill plugs are I think a 7/16 square drive, but don't hold me to that. Most people, me included, take a cheap 1/2" extension and grind it down to fit.
Before changing the differential fluid, I'd suggest checking and changing the axle oil seals, especially as a leak there can potentially end up on your rear brakes and kill them pretty quickly.
4. Check your steering rack boots. Most originals are long gone, and the replacements are at best good for a couple of years. Yours are probably bad. Replace them(tie rod ends aren't a terrible idea while you're at it), and while you're up there be sure you grease the kingpins(3 nipples on each one). Repacking the front bearings is a good idea too if you don't know when it was last done(the front one had a bad tendency to go dry). The original Timken bearings rarely go bad(they are massively overspecced for the application-they're the same as what's used on 70s full size GM cars that way 2-3x what an MGB weights), and are much higher quality than anything you're likely to find for replacement, so I'd recommend saving those. I don't have any particular grease preference, and I don't think anyone else does either-just use whatever is in your grease gun.
Note that the steering rack gets oil, not grease, which is partiallly why the gaiter boots are so critical. John Twist has a good video on Youtube about filling the steering rack as well as some adjustments to make while you're servincing this. He also has a few on servicing the front hubs-note that they're NOT set up like a typical American car(where you tighten the nut until the hub seizes and then back off). Rather, you tighten to a specific torque, back off just enough to be able to put a split through the castle nut, and check for proper float(and adjust with shims if it's not correct). See again a few John Twist videos on this.
5. Change your brake and clutch fluid if you don't know when it was last done. Really fluid-DOT3, DOT4, DOT3/4, or whatever is fine. I'd suggest using a turkey baster to suck the old fluid out of the reservoir and then refilling. As long as the MC doesn't run dry, you can be lazy and just just crack open the bleeders until they run clear to finish flushing. On brakes, you want to go left rear, right rear, right front, left front in that order. Don't let the clutch run dry as the slave cylinder can be a royal pain to bleed.
While you're back there, adjust your rear brakes. Take the wheel off(I learned that one the hard way) and turn the adjuster 1/4 turn at a time with a 1/4" wrench (you should feel a "click" with each turn) until you can't turn the drum by hand, then back off 2 clicks. It's not a terrible idea to run the adjuster screw all the way out, all the way in, and then adjust-they like to seize(I do this once a year at least on my car). If you have any residual spongy pedal feeling or whatever after you've changed the fluid and bled any potential residual air, usually adjusting the rear brakes will take that up.
BTW, watch for brake fluid leaks at the pressure differential warning activator(PDWA) valve(located on the left front fender ahead of the master cylinder-should have a big white electrical plug in the center). This is something a problematic and often misunderstood part-it has a shuttle valve in the center meant to light up the light to the left of the steering wheel on the dash if a pressure difference exists between the front and rear brakes, but it can leak, cause "cross talk" between the two circuits, or even shut one of them off completely if not reset. Mine's been modified such that it's just a glorified brake line union block, but at minimum consider at some point soon ordering a rebuild kit for the shuttle valve.
6. If you have the original SU carburetors, don't forget they need oil too
. 20W-50 engine oil works just great, and they don't take much.
If you don't have SU carbs, get them
They really are an excellent carb, and IMO the best all around option for these cars. You'll want HS4s for your car. If you do have them, know that the throttle spindles are a common and problematic wear location, and unfortunately it's really difficult to rebush these properly yourself(and not worth the money to buy the tools for a single set). Joe Curto and a few of the other carb masters will do the rebush only and return to you(along with a rebuild kit if you request). If your car is a 68 it probably has fixed FX needles, and if a 69 it's PROBABLY biased AAE needles and poppet valve throttles. If the latter, put in solid disks. I've never been happier with how my car ran than since I converted from biased to fixed needles and put #5 needles in. That's getting into the weeds, though. If your car runs fine now, tune up the carbs(there again, Twist has some good videos on this) and let them be for now unless you have a reason to get into them.
7. There's one other oil I can think of-the Armstrong shocks. This fulid needs little attention unless it's leaking, which it rarely does(unless the shock's been overfilled). It's worth giving them some attention especially since yours are closing in on 60 years, but it's easiest to do off the car and if you're pulling them, it's not a bad idea to refresh the entire axle(bushings, droplinks, etc). There are expensive solutions out there for oil, but I bought 2 gallons of hydraulic oil at Rural King(I think it had "68" in the weight? I still have the jug and can check). It was dirt cheap and 2 gallons is probably enough the service all four shocks on a dozen cars.
8. Since you asked about gasoline-your car's engine would have had a compression ratio of 8.8:1 from the factory. This is considered a "high compression" engine in factory parlance(72 and later US market engines were 8.0:1) and is a nice compromise of power and being fairly tolerant of fuel. If you set the ignition timing correctly, you should have zero issues with 87 octane. BTW, I'd highly recommend getting a dial back light if you don't have one. The manual will tell you I think 12º BTDC static. John Twist says 32º BTDC at full mechanical advance(around 4K rpms), vacuum disconnected. I don't take that as a set in stone number(even though if you do a JT rolling tech session, that's where he'll set it) but it's a great starting point and most engines will really well there.
It's worth checking your distributor, though, as a lot are pretty tired now. With a dialback light, it's fairly easy to check. I THINK your car should have a 40916 distributor, the "good" one, which should advance 20º more or less linearly from 1K to 4K RPMs(or about 5º every 750rpms). Check your vac advance can-you can do a quick check by sucking on the hose and making sure it holds vacuum. To test operation, you can use a Mightyvac connected to it. Your distributor should have a 5-13-10 vac advance. This means it will start to advance at 5" mercury, be all in at 13" mecury, and give 20º advance at the crankshaft.
I can think of one parting thought-
When I first got my car, and I've had it almost a decade now(bought it October 2015), I was excited that you could nearly build a car out of the Moss catalog. I was pretty quickly disappointed that most of what's available is junk. I buy almost exclusively through Basil Adams, a reseller out of California, who has bunches of sources and can advise a lot on quality. I also am big on refurbing old parts or fitting NOS when I can(I have a few sources I often go to for these). For the big stuff in particular, like your distributor, you're 100x letting Jeff Schlemmer rebuild it than buying a new one.
